reproduce (this is a W=1 build):
        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # install arm cross compiling tool for clang build
        # apt-get install binutils-arm-linux-gnueabi
        # clang-21 might not be officially released. Try a lower version if installation fails.
        # https://github.com/brauner/linux/commit/ac6355451deea7f96f5ca40a226fb0685b0475af
        git remote add brauner-github https://github.com/brauner/linux.git
        git fetch --no-tags brauner-github vfs-6.16.misc
        git checkout ac6355451deea7f96f5ca40a226fb0685b0475af
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang-21 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=arm olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang-21 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=arm SHELL=/bin/bash mm/