reproduce (this is a W=1 build):
        git remote add powerpc https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git
        git fetch powerpc next
        git checkout powerpc/next
        b4 shazam https://lore.kernel.org/r/b1e2171587806648500fd305918bc7e90abdeeb5.1704800524.git.christophe.leroy@csgroup.eu
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash init/