reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=e8f9d82379881461fe2d7d181f633009970bdb81
        git remote add bpf-next https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git
        git fetch --no-tags bpf-next for-next
        git checkout e8f9d82379881461fe2d7d181f633009970bdb81
        # apt-get install llvm-dev

        # save the config file
        make W=1 ARCH=i386 olddefconfig
        make W=1 ARCH=i386 
        make W=1 headers_install
        make W=1 -C tools/testing/selftests