reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/commit/?id=bdef2233bc5a3841901ce0f16afeb0f7e8f3d237
        git remote add ebiggers https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git
        git fetch --no-tags ebiggers wip-curve25519
        git checkout bdef2233bc5a3841901ce0f16afeb0f7e8f3d237
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash lib/crypto/tests/