Always use prctl(PR_SET_PTRACER)
Sufficiently old Linux kernel headers don't provide the PR_SET_PTRACER, but we can still call prctl with it if the runtime kernel is newer. Even if it's not, prctl will only return EINVAL.
Patch by Mike Hommey <firstname.lastname@example.org>
Reviewed By: eugenis
Subscribers: sylvestre.ledru, cfe-commits, kubamracek
Differential Revision: https://reviews.llvm.org/D39717