Add Xray instrumentation support to FreeBSD

Summary:
- Enabling the build.
- Using assembly for the cpuid parts.
- Using thr_self FreeBSD call to get the thread id 

Patch by: David CARLIER

Reviewers: dberris, rnk, krytarowski

Reviewed By: dberris, krytarowski

Subscribers: emaste, stevecheckoway, nglevin, srhines, kubamracek, dberris, mgorny, krytarowski, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D43278

llvm-svn: 325240