[llvm] [win][aarch64] Add support for detecting the Host CPU on Arm64 Windows (PR #151596)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> > FYI, see #130509 (which might also impact Windows with this patch).
> 
> This change isn't vital to any work that I'm doing, so if we need to back it out, then I have no problems with that.

Probably all we need to do to address this on Windows is to add `Features["sve"] = IsProcessorFeaturePresent(PF_ARM_SVE_INSTRUCTIONS_AVAILABLE);` to sys::getHostCPUFeatures.  With the minor complication that we might need to define PF_ARM_SVE_INSTRUCTIONS_AVAILABLE ourselves, like we do in compiler-rt, so LLVM compiles on older SDKs.

https://github.com/llvm/llvm-project/pull/151596