[XRay][clang] Allow clang to build XRay instrumented binaries in OpenBSD

Summary:
This patch was originally reviewed in D45126. It enables clang to add
the XRay runtime and the link-time dependencies for XRay instrumentation
in OpenBSD.

Landing for devnexen.

Reviewers: brad, dberris

Subscribers: dberris, krytarowski, cfe-commits

Author: devnexen

Differential Revision: https://reviews.llvm.org/D45126

llvm-svn: 329183