[clang] [llvm] [Reland] [PowerPC] frontend get target feature from backend with cpu name (PR #144594)

[Reid Kleckner via llvm-commits]

https://github.com/rnk approved this pull request.

Thanks, go for it.

This doesn't match the `-gen-x-target-def` pattern used for ARM, AArch64, and RISCV, but it's target-generic, which seems nice.

https://github.com/llvm/llvm-project/pull/144594