FreeBSD driver / Xray flags moving pthread to compile flags.

Summary:
- Using -lpthread instead, with -pthread the linkage does not work.
-Warning about the -fxray-instrument usage outside of the working cases.

Patch by: David CARLIER

Reviewers: krytarowski, vitalybuka, dberris, emaste

Reviewed By: krytarowski, emaste

Subscribers: srhines, emaste, cfe-commits

Differential Revision: https://reviews.llvm.org/D43378

llvm-svn: 325746