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

[Lei Huang via llvm-commits]

lei137 wrote:

Description require an update since I don't see any CPU features being set.
```
Utilize common API in PPCTargetParser (https://github.com/llvm/llvm-project/pull/97541) to set default CPU with same interfaces for LLC.
This will update AIX default CPU to pwr7 and LoP powerppc64 default CPU to ppc64.
```

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