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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

So you're saying the SVE detection is working... but we're not calling this code from clang?  Okay, that's something we can look into as a followup.

There's an existing issue report for -march=native: #149370.  There isn't any open issue for -mcpu=native, though.

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