bugfix: ignore Apple's "gcc" by default (#17589)

Apple's gcc is really clang. We previously ignored it by default but
there was a regression in #17110.

Originally we checked for all clang versions with this, but I know of
none other than `gcc` on macos that actually do this, so limiting to
`apple-clang` should be ok.

- [x] Fix check for `apple-clang` in `gcc.py` to use version detection
  from `spack.compilers.apple_clang`