reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git/commit/?id=e58cc4735e7d227ad24307bbb3f9601ee67d44f5
        git remote add tglx-devel https://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git
        git fetch --no-tags tglx-devel irq/proc
        git checkout e58cc4735e7d227ad24307bbb3f9601ee67d44f5
        # 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