reproduce (this is a W=1 build):
        # https://github.com/ClangBuiltLinux/linux/commit/21ffcd408eaea91fb594ec0a27f9108349c2d31b
        git remote add clangbuiltlinux https://github.com/ClangBuiltLinux/linux
        git fetch --no-tags clangbuiltlinux question_mark_operator
        git checkout 21ffcd408eaea91fb594ec0a27f9108349c2d31b

        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-8.5.0 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=powerpc olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-8.5.0 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=powerpc SHELL=/bin/bash mm/