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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> I could have a core that contains the SVE extension but be running a Linux kernel that does not support SVE. Purely matching the CPU name would tell you that SVE is allowed.

Yes.  Or it could be fused off, or disabled by firmware.

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