Add OpenBSD support to OpenMP
Summary: This patch permits OpenMP to build and work (with both gcc and clang) on OpenBSD. It mostly follows what was done for FreeBSD and NetBSD, except OpenBSD does not have pthread_getattr_np support, so it follows OS X in that one instance.
Reviewers: #openmp, krytarowski
Reviewed By: krytarowski
Subscribers: guansong, jfb, emaste, mgorny, krytarowski, #openmp
Differential Revision: https://reviews.llvm.org/D34280