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

[Daniel Paoliello via llvm-commits]

dpaoliello 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.
 
> Can we add any tests for this? I guess we can't really unittest registry queries, but we could at least unittest the conversion of the 32-bit integers to getHostCPUNameForARM strings.

Refactored the code a bit to improve the testing. Only thing that isn't tested is registry access (in theory we could create and mount a new hive, but that seems like overkill).

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