Add Xray instrumentation compile-time/link-time support to FreeBSD

Summary: Similarly to the GNU driver version, adding proper compile and linker flags.

Patch by: David CARLIER

Reviewers: vitalybuka, krytarowski, dberris

Reviewed By: krytarowski, dberris

Subscribers: emaste, dberris, cfe-commits

Differential Revision: https://reviews.llvm.org/D43279

llvm-svn: 325238