Rollup merge of #66548 - lenary:riscv/disable-atomics-non-a, r=alexcrichton

[RISCV] Disable Atomics on all Non-A RISC-V targets

In a `TargetOptions` configuration, `max_atomic_width: None` causes `max_atomic_width()` to return `Some(target_pointer_width)`. So, contrary to assumptions, `max_atomic_width: None` means you do have atomic support!

RISC-V's rv32i and rv32imc do not have architectural support for atomic memory accesses of any size, because they do not include the `A` architecture extension. This means the values in the target definition should be `Some(0)`.

This bug has been observed via a build failure with oreboot/oreboot#191, where LLVM was still generating libcalls for atomic operations. According to rust-lang/compiler-builtins, "Rust only exposes atomic types on platforms that support them, and therefore does not need to fall back to software implementations." - so this PR tries to bring rustc inline with this decision.

This commit also removes the outdated bug link, which references a now irrelevant GCC bug.

I will likely also have to revisit the `min_atomic_width` of all the RISC-V targets so they are correct and match what the hardware is capable of (which is more restricted than one might imagine).

r? @alexcrichton

src/librustc_target/spec/riscv32i_unknown_none_elf.rs

@@ -17,7 +17,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             linker: Some("rust-lld".to_string()),
             cpu: "generic-rv32".to_string(),
-            max_atomic_width: None,
+            max_atomic_width: Some(0),
             atomic_cas: false,
             features: String::new(),
             executables: true,

src/librustc_target/spec/riscv32imc_unknown_none_elf.rs

@@ -17,8 +17,7 @@ pub fn target() -> TargetResult {
         options: TargetOptions {
             linker: Some("rust-lld".to_string()),
             cpu: "generic-rv32".to_string(),
-            // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86005
-            max_atomic_width: None, //Some(32),
+            max_atomic_width: Some(0),
             atomic_cas: false,
             features: "+m,+c".to_string(),
             executables: true,