Update aarch64 feature detection

[mrkajetanp]

Add various aarch64 CPU features already supported by LLVM and Linux.

Linux features: https://github.com/torvalds/linux/blob/master/arch/arm64/include/uapi/asm/hwcap.h LLVM features: llvm-project/llvm/lib/Target/AArch64/AArch64.td 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_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

It also adds feature detection for FEAT_FPMR. It is somewhat of a special case because FPMR only exists as a feature in LLVM 18, it has been removed from the LLVM upstream. On that account the intention is for it to be detectable at runtime through stdarch but not have a corresponding compile-time Rust target feature.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Amanieu (or someone else) some time within the next two weeks.