[llvm] [TargetParser][AArch64] Believe runtime feature detection (PR #95694)

[via llvm-commits]

workingjubilee wrote:

This only becomes unit-testable, even for just AArch64, with a reasonable amount of effort (as opposed to the unreasonable amount I just cited) if it is acceptable to kill off the ifdefs and then expose the internal string -> StringMap parser.

But it may be a better idea to instead just switch to using HWCAP and HWCAP2?

https://github.com/llvm/llvm-project/pull/95694