FreeBSD TSan support update

Summary:
- Disable thread_finalize callback on FreeBSD, fixing couple of unit tests.

Patch by David CARLIER

Reviewers: vitalybuka

Reviewed By: vitalybuka

Subscribers: emaste, kubamracek, krytarowski, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D44156

llvm-svn: 327697