[gn build] restore build command removed in 9595a7ff for platforms without prebuilts

    platform = get_platform()

    if not platform:

        print('no prebuilt binary for', sys.platform)

        print('build it yourself with:')

        print('  rm -rf /tmp/gn &&')

        print('  pushd /tmp && git clone https://gn.googlesource.com/gn &&')

        print('  cd gn && build/gen.py && ninja -C out gn && popd &&')

        print('  mkdir -p llvm/utils/gn/bin/mac-arm64 &&')

        print('  cp /tmp/gn/out/gn somewhere/on/PATH')

        return 1

    dirname = os.path.join(os.path.dirname(__file__), 'bin', platform)

    if not os.path.exists(dirname):