Always use prctl(PR_SET_PTRACER)

Summary:
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 <mh-llvm@glandium.org>

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: sylvestre.ledru, cfe-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D39717

llvm-svn: 317668