llvm_9: patch to fix a test on older CPUs
Taken from nixpkgs master commits ea306f6dfc + 9aa8ae999ad1.
Right now it didn't build on the more loaded platforms (x86_64-*)
https://hydra.nixos.org/eval/1676683
and on x86_64-linux that breaks very many builds.
(wendy machine is being too active, so restarts don't help)
