reproduce (this is a W=1 build):
        # https://github.com/davidhildenbrand/linux/commit/06e0fc4651a486ac1ce333cccc89b394222431c8
        git remote add davidhildenbrand https://github.com/davidhildenbrand/linux
        git fetch --no-tags davidhildenbrand nr_cpus
        git checkout 06e0fc4651a486ac1ce333cccc89b394222431c8
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um SUBARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=um SUBARCH=i386 SHELL=/bin/bash arch/um/os-Linux/skas/