Rollup merge of #128192 - mrkajetanp:feature-detect, r=Amanieu

rustc_target: Add various aarch64 features

Add various aarch64 features already supported by LLVM and Linux.
Additionally include some comment fixes to ensure consistency of feature names with the Arm ARM.
Compiler support for features added to stdarch by https://github.com/rust-lang/stdarch/pull/1614.
Tracking issue for unstable aarch64 features is https://github.com/rust-lang/rust/issues/127764.

List of added features:

- FEAT_CSSC
- FEAT_ECV
- FEAT_FAMINMAX
- FEAT_FLAGM2
- FEAT_FP8
- FEAT_FP8DOT2
- FEAT_FP8DOT4
- FEAT_FP8FMA
- FEAT_HBC
- FEAT_LSE128
- FEAT_LSE2
- FEAT_LUT
- FEAT_MOPS
- FEAT_LRCPC3
- FEAT_SVE_B16B16
- FEAT_SVE2p1
- FEAT_WFxT
- FEAT_SME
- FEAT_SME_F16F16
- FEAT_SME_F64F64
- FEAT_SME_F8F16
- FEAT_SME_F8F32
- FEAT_SME_FA64
- FEAT_SME_I16I64
- FEAT_SME_LUTv2
- FEAT_SME2
- FEAT_SME2p1
- FEAT_SSVE_FP8DOT2
- FEAT_SSVE_FP8DOT4
- FEAT_SSVE_FP8FMA

FEAT_FPMR is added in the first commit and then removed in a separate one to highlight it being removed from upstream LLVM 19. The intention is for it to be detectable at runtime through stdarch but not have a corresponding Rust compile-time feature.

compiler/rustc_feature/src/unstable.rs

@@ -302,6 +302,7 @@ declare_features! (
     // FIXME: Document these and merge with the list below.
 
     // Unstable `#[target_feature]` directives.
+    (unstable, aarch64_unstable_target_feature, "CURRENT_RUSTC_VERSION", Some(44839)),
     (unstable, aarch64_ver_target_feature, "1.27.0", Some(44839)),
     (unstable, arm_target_feature, "1.27.0", Some(44839)),
     (unstable, avx512_target_feature, "1.27.0", Some(44839)),