Add ability to infer CHPL_LLVM_GCC_INSTALL_DIR

[jabraham17]

Adds the ability to infer CHPL_LLVM_GCC_INSTALL_DIR. Resolves #25925

This PR uses similar logic to the LLVM spack package to determine this. Thanks @PHHargrove for the pointer to this.

Testing

• A system with a "broken" clang can fix itself automatically now
  • relatedly, a user can request no gcc install dir with CHPL_LLVM_GCC_INSTALL_DIR=none
• make check on a nightly test system
• full paratest with/without gasnet on linux64
• make check on M1 mac
• Setting GCC_INSTALL_DIR or GCC_PREFIX turns off inference
• Setting both GCC_INSTALL_DIR and GCC_PREFIX warns, then prefers GCC_INSTALL_DIR if available
• If supported, GCC_INSTALL_DIR is preferred (unless user requests otherwise)

This PR also adds a missing line to overrides.py for CHPL_LLVM_GCC_INSTALL_DIR

[Reviewed by @mppf]

[mppf]

I would assume that we want this to replace the by-default inference in get_gcc_prefix_dir. That is, if LLVM version is new enough, we should infer CHPL_LLVM_GCC_INSTALL_DIR but not CHPL_LLVM_GCC_PREFIX. If LLVM is too old, we should infer CHPL_LLVM_GCC_PREFIX but not CHPL_LLVM_GCC_INSTALL_DIR.

Also, presumably, we want to infer neither of these if one of them is set in the environment.

[jabraham17]

I would assume that we want this to replace the by-default inference in get_gcc_prefix_dir. That is, if LLVM version is new enough, we should infer CHPL_LLVM_GCC_INSTALL_DIR but not CHPL_LLVM_GCC_PREFIX. If LLVM is too old, we should infer CHPL_LLVM_GCC_PREFIX but not CHPL_LLVM_GCC_INSTALL_DIR

That should be the logic already. This PR didn't touch the existing logic, which already prefers GCC_INSTALL_DIR

    gcc_install_dir = get_gcc_install_dir()
    if gcc_install_dir:
        clang_args.append('--gcc-install-dir=' + gcc_install_dir)
    else:
        gcc_prefix = get_gcc_prefix_dir(clang_cfg_args)
        if gcc_prefix:
            clang_args.append('--gcc-toolchain=' + gcc_prefix)

Also, presumably, we want to infer neither of these if one of them is set in the environment.

This is currently not handled. Also, if both are specified I expect we would want to prefer GCC_INSTALL_DIR and warn about GCC_PREFIX being set. I will make add this

[bradcray]

Ooh, exciting!