[clang] [clang][powerpc][wasm][systemz][x86] Replace target vector popcount intrinsics with __builtin_elementwise_popcount (PR #109160)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

Officially we don't guarantee target intrinsics at all - but I can add a release note if it will help?

https://github.com/llvm/llvm-project/pull/109160