reproduce (this is a W=1 build):
        mkdir -p ~/bin
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install um cross compiling tool for clang build
        # apt-get install binutils-um-linux-gnu
        # https://git.kernel.org/pub/scm/linux/kernel/git/jlayton/linux.git/commit/?id=57758153d414f883fcc208930e2b0911bc6b50e3
        git remote add jlayton https://git.kernel.org/pub/scm/linux/kernel/git/jlayton/linux.git
        git fetch --no-tags jlayton ctime-next
        git checkout 57758153d414f883fcc208930e2b0911bc6b50e3
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang ~/bin/make.cross W=1 O=build_dir ARCH=um olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang ~/bin/make.cross W=1 O=build_dir ARCH=um SHELL=/bin/bash fs/