[sanitizer] Define __sanitizer_clockid_t on FreeBSD

Summary:
https://reviews.llvm.org/D41121 broke the FreeBSD build due to that type not
being defined on FreeBSD. As far as I can tell, it is an int, but I do not have
a way to test the change.

Reviewers: alekseyshl, kparzysz

Reviewed By: kparzysz

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

Differential Revision: https://reviews.llvm.org/D41325

llvm-svn: 320931