reproduce (this is a W=1 build):
        # https://github.com/Rust-for-Linux/linux/commit/cfd66f531af29e0616c58b4cd4c72770a5ac4081
        git remote add rust https://github.com/Rust-for-Linux/linux
        git fetch --no-tags rust staging/dev
        git checkout cfd66f531af29e0616c58b4cd4c72770a5ac4081
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um olddefconfig
        make W=1 O=build_dir ARCH=um prepare