[OMPT] Fix duplicate implicit_task_end events for master thread with GCC

For non-serialized parallel regions the master thread issued two callbacks:
The first one in kmp_gsupport.c and the second in __kmp_join_call. Therefore
only trigger the callback in kmp_gsupport.c for serialized parallel regions.

Differential Revision: http://reviews.llvm.org/D16716

llvm-svn: 264264