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