Support linking against OpenMP runtime on FreeBSD.

Summary:
Similar to rL248426 (which was a followup to rL248379 and rL248424), add the
required libraries for OpenMP on the linker command line, and update the test
case.

Reviewers: emaste, theraven, joerg

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D13822

llvm-svn: 250657