Rollup merge of #132552 - taiki-e:sparc-target-feature, r=workingjubilee

Add v9, v8plus, and leoncasa target feature to sparc and use v8plus in create_object_file

This adds the following three unstable target features:

- `v9`: SPARC-V9 instructions ([LLVM definition][sparc-v9])
  - Relevant to https://github.com/rust-lang/rust/pull/131222#issuecomment-2453310963
  - Relevant to https://github.com/rust-lang/rust/pull/132472#discussion_r1832606081
  - This is also needed to implement https://github.com/taiki-e/atomic-maybe-uninit/pull/31 (depends on inline assembly support) more robustly.
- `v8plus`: SPARC-V8+ ABI ([LLVM definition][sparc-v8plus])
  - This is added in LLVM 20. In LLVM 19 and older, it is emulated to work the same way as LLVM in each LLVM version.
  - See https://github.com/rust-lang/rust/issues/132585#issuecomment-2453926257 for more.
- `leoncasa`: CASA instruction[^1] of LEON3 and LEON4 processors ([LLVM definition][sparc-leoncasa], LLVM feature name: `hasleoncasa`)
  - This is needed to implement https://github.com/taiki-e/atomic-maybe-uninit/pull/31 (depends on inline assembly support) more robustly.

[^1]: Atomic CAS instruction

[sparc-v9]: f5e4ffaa49/llvm/lib/Target/Sparc/Sparc.td (L37-L39)
[sparc-v8plus]: f5e4ffaa49/llvm/lib/Target/Sparc/Sparc.td (L37-L39)
[sparc-leoncasa]: https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/Sparc/LeonFeatures.td#L32-L37

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -212,7 +212,7 @@ pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static
         "riscv32" => (Architecture::Riscv32, None),
         "riscv64" => (Architecture::Riscv64, None),
         "sparc" => {
-            if sess.target.options.cpu == "v9" {
+            if sess.unstable_target_features.contains(&sym::v8plus) {
                 // Target uses V8+, aka EM_SPARC32PLUS, aka 64-bit V9 but in 32-bit mode
                 (Architecture::Sparc32Plus, None)
             } else {