reproduce (this is a W=1 build):
        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # install um cross compiling tool for clang build
        # apt-get install binutils-um-linux-gnu
        # https://github.com/intel/linux-intel-lts/commit/be80de30b347dc2eae35dd5f7905ad9c9167dc08
        git remote add intel-lts https://github.com/intel/linux-intel-lts.git
        git fetch --no-tags intel-lts 6.6/preempt-rt
        git checkout be80de30b347dc2eae35dd5f7905ad9c9167dc08
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang-21 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=um olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang-21 ~/lkp-tests/kbuild/make.cross W=1 O=build_dir ARCH=um SHELL=/bin/bash fs/hostfs/