Minimize delay between sem_post and sigsuspend in suspend_handler if TSan

Minimize delay between sem_post and sigsuspend in suspend_handler if TSan
(fix of commit af409e4)

Issue #181 (bdwgc).

This change is to do as less as possible (even in case of TSan usage)
between the sem_post and sigsuspend calls in GC_suspend_handler_inner
(to match the relevant comment after sigsuspend call).

* pthread_stop_world.c [!GC_OPENBSD_UTHREADS && !NACL
&& THREAD_SANITIZER] (GC_suspend_handler_inner): Move sigemptyset()
and pthread_sigmask() calls to be just before sem_post() call.