reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?id=6b83afdcfa935db4f836906ca7f0fcafdca8d229
        git remote add akpm-mm https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git
        git fetch --no-tags akpm-mm mm-unstable
        git checkout 6b83afdcfa935db4f836906ca7f0fcafdca8d229

        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # apt-get install llvm-dev

        # save the config file
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-15.1.0 ~/lkp-tests/kbuild/make.cross W=1 ARCH=powerpc olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-15.1.0 ~/lkp-tests/kbuild/make.cross W=1 ARCH=powerpc 
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-15.1.0 ~/lkp-tests/kbuild/make.cross W=1 headers
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-15.1.0 ~/lkp-tests/kbuild/make.cross W=1 -C tools/testing/selftests