Fix 'Bad signal in suspend_handler' abort on FreeBSD-9.2
(Apply commit e741b45 from 'master' branch.)

* pthread_stop_world.c (GC_suspend_handler_inner): If signal number
is zero then silently return (instead of ABORT) if FREEBSD (to
workaround a bug in FreeBSD 9.2-RELEASE regarding signals processing,
bug is observed only if THREAD_LOCAL_ALLOC defined).

Conflicts:

    pthread_stop_world.c