reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git/commit/?id=0bdad28369fc5e93de39b5046228ed78e982fc71
        git remote add masahiroy-kbuild https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git
        git fetch --no-tags masahiroy-kbuild kbuild
        git checkout 0bdad28369fc5e93de39b5046228ed78e982fc71
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash kernel/