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

[zhijian lin via llvm-commits]

diggerlin wrote:

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

If I add an NFC patch, it can be difficult for other developers to understand why it's needed. By explicitly including -mcpu=ppc in the patch, it becomes clear that this addition is due to the default CPU change in llc.

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