[llvm] [AArch64] Improve host feature detection. (PR #160410)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

The linux part LGTM.

(Like you said I think in the long run we might want to take what hwcaps says and trust it for all features we know about. I believe that is how GCC handles it, but there might be some subtlety in the details of which to use in what way).

https://github.com/llvm/llvm-project/pull/160410