reproduce (this is a W=1 build):
        git clone https://github.com/intel/lkp-tests.git ~/lkp-tests
        # https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/commit/?id=44bb30a0a76203af426651795ff0c90bfc60bdc0
        git remote add vbabka https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git
        git fetch --no-tags vbabka slub-percpu-sheaves-v6-benchmarking
        git checkout 44bb30a0a76203af426651795ff0c90bfc60bdc0
        # 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