reproduce (this is a W=1 build):
        mkdir -p ~/bin
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install um cross compiling tool for clang build
        # apt-get install binutils-um-linux-gnu
        # https://github.com/jcmvbkbc/linux-xtensa/commit/20b0bc011b5325431d41d666f8f949d0c6ede432
        git remote add jcmvbkbc-xtensa https://github.com/jcmvbkbc/linux-xtensa
        git fetch --no-tags jcmvbkbc-xtensa xtensa-6.5-esp32
        git checkout 20b0bc011b5325431d41d666f8f949d0c6ede432
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang ~/bin/make.cross W=1 O=build_dir ARCH=um olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang ~/bin/make.cross W=1 O=build_dir ARCH=um SHELL=/bin/bash