tsan: fix darwin build after 328079

328079 introduced a weak hook without default implementation.
This broke darwin build:
http://green.lab.llvm.org/green//job/clang-stage1-configure-RA/43731/consoleFull#-119213188149ba4694-19c4-4d7e-bec5-911270d8a58c

Provide default impl for the hook.

llvm-svn: 328082