[clang/mac] Make -mmacos-version-min the canonical spelling over -mmacosx-version-min

This was promised 5 years ago in https://reviews.llvm.org/D32796,
let's do it.

Both flags are still accepted. No behavior change except for which
form shows up in --help output and in dumps of internal state
(such as with RC_DEBUG_OPTIONS).

Differential Revision: https://reviews.llvm.org/D129226