[XRay][compiler-rt] Build XRay runtime for OpenBSD

Summary:
This is D45125; the patch enables the build of XRay on OpenBSD. We also
introduce some OpenBSD specific changes to the runtime implementation,
involving how we get the TSC rate through the syscall interface specific
to OpenBSD.

Reviewers: dberris

Authored by: devnexen

Subscribers: dberris, mgorny, krytarowski, llvm-commits

Differential Revision: https://reviews.llvm.org/D45125

llvm-svn: 329189