reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?id=db6fe4d61ece24193eb4d94a82d967501d53358c
        git remote add kees https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git
        git fetch --no-tags kees for-next/move-kunit-tests
        git checkout db6fe4d61ece24193eb4d94a82d967501d53358c
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um olddefconfig
        make W=1 O=build_dir ARCH=um SHELL=/bin/bash