Rollup merge of #92383 - lancethepants:armv7-unknown-linux-uclibceabi, r=nagisa

Add new target armv7-unknown-linux-uclibceabi (softfloat)

This adds the new target `armv7-unknown-linux-uclibceabi (softfloat)`. It is of course similar to `armv7-unknown-linux-uclibceabihf (hardfloat)` which was just recently added to rust except that it is `softfloat`.

My interest lies in the Broadcom BCM4707/4708/BCM4709 family, notably found in some Netgear and Asus consumer routers. The armv7 Cortex-A9 cpus found in these devices do not have an fpu or NEON support.

With this patch I've been able to bootstrap rustc, std and host tools `(extended = true)` to run on the target device for native compilation, allowing the target to be used as a development platform.

With the recent addition of `armv7-unknown-linux-uclibceabihf (hardfloat)` it looks like many of the edge cases of using the uclibc c-library are getting worked out nicely. I've been able to compile some complex projects. Some patching still needed in some crates, but getting there for sure.  I think `armv7-unknown-linux-uclibceabi` is ready to be a tier 3 target.

I use a cross-toolchain from my project to bootstrap rust.
https://github.com/lancethepants/tomatoware
The goal of this project is to create a native development environment with support for various languages.

compiler/rustc_target/src/spec/armv7_unknown_linux_uclibceabi.rs

@@ -0,0 +1,23 @@
+use crate::spec::{Target, TargetOptions};
+
+// This target is for uclibc Linux on ARMv7 without NEON,
+// thumb-mode or hardfloat.
+
+pub fn target() -> Target {
+    let base = super::linux_uclibc_base::opts();
+    Target {
+        llvm_target: "armv7-unknown-linux-gnueabi".to_string(),
+        pointer_width: 32,
+        data_layout: "e-m:e-p:32:32-Fi8-i64:64-v128:64:128-a:0:32-n32-S64".to_string(),
+        arch: "arm".to_string(),
+
+        options: TargetOptions {
+            features: "+v7,+thumb2,+soft-float,-neon".to_string(),
+            cpu: "generic".to_string(),
+            max_atomic_width: Some(64),
+            mcount: "_mcount".to_string(),
+            abi: "eabi".to_string(),
+            ..base
+        },
+    }
+}

compiler/rustc_target/src/spec/mod.rs

@@ -1013,6 +1013,7 @@ supported_targets! {
 
     ("armv6k-nintendo-3ds", armv6k_nintendo_3ds),
 
+    ("armv7-unknown-linux-uclibceabi", armv7_unknown_linux_uclibceabi),
     ("armv7-unknown-linux-uclibceabihf", armv7_unknown_linux_uclibceabihf),
 
     ("x86_64-unknown-none", x86_64_unknown_none),