Fix logic/compile errors in "event callbacks" code (Darwin, Pthreads)

Fix logic/compile errors in "event callbacks" code (Darwin, Pthreads)

* darwin_stop_world.c (GC_stop_world): Replace
GC_EVENT_THREAD_UNSUSPENDED to GC_EVENT_THREAD_SUSPENDED.
* pthread_stop_world.c (GC_on_collection_event): Declare.
* pthread_stop_world.c (GC_suspend_all): Change type of "thread_id"
local variable to pthread_t (or pid_t for Android) instead of int; fix
"threadid" parameter to "thread_id" in GC_on_collection_event call;
implement notification for NaCl target.
* pthread_stop_world.c (GC_start_world): Declare "thread_id" local
variable; implement notification for NaCl target.