[TSan] Initialize libdispatch interceptors if necessary

On Linux (and other non-Darwin platforms) we need to initialize
interceptors. Since tsan_libdispatch.cc is compiled optionally, add a
weak default implementation of `InitializeLibdispatchInterceptors`.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D59113

llvm-svn: 355717