Fix ABI detection issues with macOS gcc. (#3854)

- gcc on macOS says it's version 4.2.1, but it's really clang, and it's
  actually the *same* clang as the system clang.

- It also doesn't respond with a full path when called with
  --print-file-name=libstdc++.dylib, which is expected from gcc in abi.py.
  Instead, it gives a relative path and _gcc_compiler_compare doesn't
  understand what to do with it.  This results in errors like:

  ```
  lib/spack/spack/abi.py, line 71, in _gcc_get_libstdcxx_version
      libpath = os.readlink(output.strip())
  OSError: [Errno 2] No such file or directory: 'libstdc++.dylib'
  ```

- This commit does two things:

  1. Ignore any gcc that's actually clang in abi.py.  We can probably do
     better than this, but it's not clear there is a need to, since we
     should handle the compiler as clang, not gcc.

  2. Don't auto-detect any "gcc" that is actually clang anymore.  Ignore
     it and expect people to use clang (which is the default macOS
     compiler anyway).

Users can still add fake gccs to their compilers.yaml if they want, but
it's discouraged.