[llvm] [PowerPC][LLC] Utilize PPC::getNormalizedPPCTargetCPU() to set CPU (PR #113943)

[David Tenty via llvm-commits]

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

LGTM, thanks.

Maybe the change to make `powerpc64-unknown-unknown` default to `ppc64` generic CPU instead of the `ppc` generic CPU is worth documenting as a release note

https://github.com/llvm/llvm-project/pull/113943