reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6d9500bb1ff8c7f9c3ce199521c41aa41e8fd994
        git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
        git fetch --no-tags linus master
        git checkout 6d9500bb1ff8c7f9c3ce199521c41aa41e8fd994
        # apt-get install llvm-dev

        # save the config file
        make W=1 ARCH=x86_64 olddefconfig
        make W=1 ARCH=x86_64 
        make W=1 headers
        make W=1 -C tools/testing/selftests