[TSan] FreeBSD / intercept thr_exit

intercepting thr_exit to terminate threads under FreeBSD.
Unblock few unit tests hanging.

Reviewers: krytarowski, vitalybuka, emaste

Reviewed By: dvyukov		

Differential Revision: https://reviews.llvm.org/D47677

M    lib/tsan/rtl/tsan_interceptors.cc

llvm-svn: 333870