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

[Chen Zheng via cfe-commits]

https://github.com/chenzheng1030 approved this pull request.

LGTM except one nit. Thanks very much for adding this support.

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