[clang] [compiler-rt] [llvm] [FMV][AArch64] Remove features which expose non exploitable runtime behavior. (PR #114387)

[Alexandros Lamprineas via cfe-commits]

labrinea wrote:

> I wonder if we can get any cases of using asm msr in function version for features in question? Anyway LGTM to be aligned with specification.

Dit was the only FMV feature with a corresponding PSTATE value which could be dirrectly used with MRS/MSR-immediate. The rest of the features have corresponding bits in FPCR and SCTLR_ELx. Those registers can be accessed in the absence of rpres, ebf16, and memtag3, that's why we don't have a Subtarget feature for them in the LLVM backend.

https://github.com/llvm/llvm-project/pull/114387