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

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D34280

llvm-svn: 348726