-fzero-call-used-regs=used-gpr

[nickdesaulniers]

gcc is adding a way to specify that certain registers should be zero'd. Let's see if llvm will entertain such a feature.

gcc patch: https://github.com/clearlinux-pkgs/gcc/blob/master/zero-regs-gcc8.patch
llvm feature request: https://llvm.org/pr37880

@kees