[clang] [llvm] [PowerPC] Add an alias for -mregnames so that full register names used in assembly. (PR #70255)

[Nick Desaulniers via llvm-commits]

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

LGTM; might be nice if it's possible to set this for `opt`/`llc` invocations, but for now I'm just happy to have this in clang.

https://github.com/llvm/llvm-project/pull/70255