Rollup merge of #100415 - WorksButNotTested:be8, r=wesleywiser

Add BE8 support

Built using the following `/config.toml`
```
changelog-seen = 2

[llvm]
download-ci-llvm = false
skip-rebuild = true
optimize = true
ninja = true
targets = "ARM;X86"
clang = false

[build]
target = ["x86_64-unknown-linux-gnu", "armeb-linux-gnueabi"]
docs = false
docs-minification = false
compiler-docs = false
[install]
prefix = "/home/user/x-tools/rust/"

[rust]
debug-logging=true
backtrace = true
incremental = true

[target.x86_64-unknown-linux-gnu]

[dist]

[target.armeb-linux-gnueabi]
cc = "/home/user/x-tools/armeb-linux-gnueabi/bin/armeb-linux-gnueabi-gcc"
cxx = "/home/user/x-tools/armeb-linux-gnueabi/bin/armeb-linux-gnueabi-g++"
ar = "/home/user/x-tools/armeb-linux-gnueabi/bin/armeb-linux-gnueabi-ar"
ranlib = "/home/user/x-tools/armeb-linux-gnueabi/bin/armeb-linux-gnueabi-ranlib"
linker = "/home/user/x-tools/armeb-linux-gnueabi/bin/armeb-linux-gnueabi-gcc"
llvm-config = "/home/user/x-tools/clang/bin/llvm-config"
llvm-filecheck = "/home/user/x-tools/clang/bin/FileCheck"
```

The following `.cargo/config` is needed inside any project directory:
```
[build]
target = "armeb-linux-gnueabi"

[target.armeb-linux-gnueabi]
linker = "armeb-linux-gnueabi-gcc"
```

compiler/rustc_target/src/spec/armeb_unknown_linux_gnueabi.rs

@@ -0,0 +1,19 @@
+use crate::abi::Endian;
+use crate::spec::{Target, TargetOptions};
+
+pub fn target() -> Target {
+    Target {
+        llvm_target: "armeb-unknown-linux-gnueabi".into(),
+        pointer_width: 32,
+        data_layout: "E-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64".into(),
+        arch: "arm".into(),
+        options: TargetOptions {
+            abi: "eabi".into(),
+            features: "+strict-align,+v8,+crc".into(),
+            endian: Endian::Big,
+            max_atomic_width: Some(64),
+            mcount: "\u{1}__gnu_mcount_nc".into(),
+            ..super::linux_gnu_base::opts()
+        },
+    }
+}

compiler/rustc_target/src/spec/mod.rs

@@ -932,6 +932,7 @@ supported_targets! {
     ("sparc64-unknown-linux-gnu", sparc64_unknown_linux_gnu),
     ("arm-unknown-linux-gnueabi", arm_unknown_linux_gnueabi),
     ("arm-unknown-linux-gnueabihf", arm_unknown_linux_gnueabihf),
+    ("armeb-unknown-linux-gnueabi", armeb_unknown_linux_gnueabi),
     ("arm-unknown-linux-musleabi", arm_unknown_linux_musleabi),
     ("arm-unknown-linux-musleabihf", arm_unknown_linux_musleabihf),
     ("armv4t-unknown-linux-gnueabi", armv4t_unknown_linux_gnueabi),

src/doc/rustc/src/SUMMARY.md

@@ -18,6 +18,7 @@
     - [aarch64-apple-ios-sim](platform-support/aarch64-apple-ios-sim.md)
     - [\*-apple-watchos\*](platform-support/apple-watchos.md)
     - [aarch64-nintendo-switch-freestanding](platform-support/aarch64-nintendo-switch-freestanding.md)
+    - [armeb-unknown-linux-gnueabi](platform-support/armeb-unknown-linux-gnueabi.md)
     - [armv4t-none-eabi](platform-support/armv4t-none-eabi.md)
     - [armv6k-nintendo-3ds](platform-support/armv6k-nintendo-3ds.md)
     - [armv7-unknown-linux-uclibceabi](platform-support/armv7-unknown-linux-uclibceabi.md)

src/doc/rustc/src/platform-support.md

@@ -223,6 +223,7 @@ target | std | host | notes
 `aarch64_be-unknown-linux-gnu_ilp32` | ✓ | ✓ | ARM64 Linux (big-endian, ILP32 ABI)
 `aarch64_be-unknown-linux-gnu` | ✓ | ✓ | ARM64 Linux (big-endian)
 [`arm64_32-apple-watchos`](platform-support/apple-watchos.md) | ✓ | | ARM Apple WatchOS 64-bit with 32-bit pointers
+[`armeb-unknown-linux-gnueabi`](platform-support/armeb-unknown-linux-gnueabi.md) | ✓ | ? | ARM BE8 the default ARM big-endian architecture since [ARMv6](https://developer.arm.com/documentation/101754/0616/armlink-Reference/armlink-Command-line-Options/--be8?lang=en).
 `armv4t-none-eabi` | * |  | ARMv4T A32
 `armv4t-unknown-linux-gnueabi` | ? |  |
 `armv5te-unknown-linux-uclibceabi` | ? |  | ARMv5TE Linux with uClibc

src/doc/rustc/src/platform-support/armeb-unknown-linux-gnueabi.md

@@ -0,0 +1,74 @@
+# armeb-unknown-linux-gnueabi
+**Tier: 3**
+
+Target for cross-compiling Linux user-mode applications targetting the ARM BE8 architecture.
+
+## Overview
+BE8 architecture retains the same little-endian ordered code-stream used by conventional little endian ARM systems, however the data accesses are in big-endian. BE8 is used primarily in high-performance networking applications where the ability to read packets in their native "Network Byte Order" is important (many network protocols transmit data in big-endian byte order for their wire formats).
+
+## History
+BE8 architecture is the default big-endian architecture for ARM since [ARMv6](https://developer.arm.com/documentation/101754/0616/armlink-Reference/armlink-Command-line-Options/--be8?lang=en). It's predecessor, used for ARMv4 and ARMv5 devices was [BE32](https://developer.arm.com/documentation/dui0474/j/linker-command-line-options/--be32). On ARMv6 architecture, endianness can be configured via [system registers](https://developer.arm.com/documentation/ddi0290/g/unaligned-and-mixed-endian-data-access-support/mixed-endian-access-support/interaction-between-the-bus-protocol-and-the-core-endianness). However, BE32 was withdrawn for [ARMv7](https://developer.arm.com/documentation/ddi0406/cb/Appendixes/Deprecated-and-Obsolete-Features/Obsolete-features/Support-for-BE-32-endianness-model) onwards.
+
+## Target Maintainers
+* [@WorksButNotTested](https://github.com/WorksButNotTested)
+
+## Requirements
+The target is cross-compiled. This target supports `std` in the normal way (indeed only nominal changes are required from the standard ARM configuration).
+
+## Target definition
+The target definition can be seen [here](https://github.com/rust-lang/rust/tree/master/compiler/rustc_target/src/spec/armeb_unknown_linux_gnueabi.rs). In particular, it should be noted that the `features` specify that this target is built for the ARMv8 core. Though this can likely be modified as required.
+
+## Building the target
+Because it is Tier 3, rust does not yet ship pre-compiled artifacts for this target.
+
+Therefore, you can build Rust with support for the target by adding it to the target list in config.toml, a sample configuration is shown below. It is expected that the user already have a working GNU compiler toolchain and update the paths accordingly.
+
+```toml
+[llvm]
+download-ci-llvm = false
+skip-rebuild = true
+optimize = true
+ninja = true
+targets = "ARM;X86"
+clang = false
+
+[build]
+target = ["x86_64-unknown-linux-gnu", "armeb-unknown-linux-gnueabi"]
+docs = false
+docs-minification = false
+compiler-docs = false
+[install]
+prefix = "/home/user/x-tools/rust/"
+
+[rust]
+debug-logging=true
+backtrace = true
+incremental = true
+
+[target.x86_64-unknown-linux-gnu]
+
+[dist]
+
+[target.armeb-unknown-linux-gnueabi]
+cc = "/home/user/x-tools/armeb-unknown-linux-gnueabi/bin/armeb-unknown-linux-gnueabi-gcc"
+cxx = "/home/user/x-tools/armeb-unknown-linux-gnueabi/bin/armeb-unknown-linux-gnueabi-g++"
+ar = "/home/user/x-tools/armeb-unknown-linux-gnueabi/bin/armeb-unknown-linux-gnueabi-ar"
+ranlib = "/home/user/x-tools/armeb-unknown-linux-gnueabi/bin/armeb-unknown-linux-gnueabi-ranlib"
+linker = "/home/user/x-tools/armeb-unknown-linux-gnueabi/bin/armeb-unknown-linux-gnueabi-gcc"
+llvm-config = "/home/user/x-tools/clang/bin/llvm-config"
+llvm-filecheck = "/home/user/x-tools/clang/bin/FileCheck"
+```
+
+## Building Rust programs
+
+The following `.cargo/config` is needed inside any project directory to build for the BE8 target:
+
+```toml
+[build]
+target = "armeb-unknown-linux-gnueabi"
+
+[target.armeb-unknown-linux-gnueabi]
+linker = "armeb-unknown-linux-gnueabi-gcc"
+```
+
+Note that it is expected that the user has a suitable linker from the GNU toolchain.