reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/broonie/misc.git/commit/?id=64bdef3832272bcbfafd7ff13aa1113fb12c8612
        git remote add broonie-misc https://git.kernel.org/pub/scm/linux/kernel/git/broonie/misc.git
        git fetch --no-tags broonie-misc arm64-gcs
        git checkout 64bdef3832272bcbfafd7ff13aa1113fb12c8612
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um SUBARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=um SUBARCH=i386 SHELL=/bin/bash mm/