[clang] [llvm] [PowerPC] Add support for -mcpu=pwr11 / -mtune=pwr11 (PR #99511)

[Chen Zheng via llvm-commits]

https://github.com/chenzheng1030 commented:

We also need more tests like:
1: `llvm/test/CodeGen/PowerPC/check-cpu.ll`
2: at least one test in the backend like `llvm/test/CodeGen/PowerPC/p10-constants.ll`?

https://github.com/llvm/llvm-project/pull/99511