Really prevent hang in sem_wait in stop_world if compiled with TSan

Really prevent hang in sem_wait in stop_world if compiled with TSan
(fix commit af409e4)

Issue #181 (bdwgc).

* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL
&& THREAD_SANITIZER] (GC_retry_signals): Initialize to TRUE.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL]
(GC_store_stack_ptr): Update comment.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL
&& GC_ENABLE_SUSPEND_THREAD] (GC_suspend_thread): Likewise.
* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL
&& THREAD_SANITIZER] (GC_stop_world): Do not use sem_trywait+sleep
workaround (not needed if GC_retry_signals).