[llvm] [clang] [PowerPC][X86] Make cpu id builtins target independent and lower for PPC (PR #68919)

[Amy Kwan via cfe-commits]

https://github.com/amy-kwan commented:

I think I primarily have minor nit comments. Thank you for the update, Nemanja.

https://github.com/llvm/llvm-project/pull/68919