rustc_target: Add various aarch64 features

Add various aarch64 features already supported by LLVM and Linux.

The features are marked as unstable using a newly added symbol, i.e.
aarch64_unstable_target_feature.

Additionally include some comment fixes to ensure consistency of
feature names with the Arm ARM and support for architecture version
target features up to v9.5a.

This commit adds compiler support for the following features:

- FEAT_CSSC
- FEAT_ECV
- FEAT_FAMINMAX
- FEAT_FLAGM2
- FEAT_FP8
- FEAT_FP8DOT2
- FEAT_FP8DOT4
- FEAT_FP8FMA
- FEAT_FPMR
- FEAT_HBC
- FEAT_LSE128
- FEAT_LSE2
- FEAT_LUT
- FEAT_MOPS
- FEAT_LRCPC3
- FEAT_SVE_B16B16
- FEAT_SVE2p1
- FEAT_WFxT

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -234,6 +234,8 @@ pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> LLVMFeature<'a
         ("aarch64", "pmuv3") => LLVMFeature::new("perfmon"),
         ("aarch64", "paca") => LLVMFeature::new("pauth"),
         ("aarch64", "pacg") => LLVMFeature::new("pauth"),
+        ("aarch64", "sve-b16b16") => LLVMFeature::new("b16b16"),
+        ("aarch64", "flagm2") => LLVMFeature::new("altnzcv"),
         // Rust ties fp and neon together.
         ("aarch64", "neon") => {
             LLVMFeature::with_dependency("neon", TargetFeatureFoldStrength::Both("fp-armv8"))