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

[Lei Huang via llvm-commits]

https://github.com/lei137 commented:

I think it's better to split this into 2 patches.  
1. update tc to add -mcpu-ppc explicitly in prep for this change.
2. current code and relevant tc update + release note update.

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