[Lsan] intercept thr_exit on FreeBSD

Intercepts thr_exit call on FreeBSD.
Disable pthread key workflow.
The pthread key create approach does not function under FreeBSD as the libpthread is not initialised enough at this stage.

Reviewers: vitalybuka, krytarowski, dim

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D48268

llvm-svn: 335164