Fix GC_thread_is_registered for finished threads

(a cherry-pick of commit 5a8c028 from 'master')

A finished thread may still stay in GC_threads, but it is no longer
considered during a garbage collection phase, thus
GC_thread_is_registered() should return false in such a case.

* include/gc.h [GC_THREADS] (GC_thread_is_registered): Refine
comment (describe the case when the thread is marked as finished).
* pthread_support.c (GC_thread_is_registered): Return FALSE if
me->flags&FINISHED.
* win32_threads.c (GC_thread_is_registered): Return FALSE if
KNOWN_FINISHED(me).

include/gc.h

@@ -1577,6 +1577,8 @@ GC_API void GC_CALL GC_start_mark_threads(void);
 
   /* Return non-zero (TRUE) if and only if the calling thread is        */
   /* registered with the garbage collector.                             */
+  /* If the thread is finished (e.g. running in a destructor and not    */
+  /* registered manually again), it is considered as not registered.    */
   GC_API int GC_CALL GC_thread_is_registered(void);
 
   /* Notify the collector about the stack and the alt-stack of the      */

pthread_support.c

@@ -761,7 +761,7 @@ GC_API int GC_CALL GC_thread_is_registered(void)
     LOCK();
     me = GC_lookup_thread(self);
     UNLOCK();
-    return me != NULL;
+    return me != NULL && !(me -> flags & FINISHED);
 }
 
 static pthread_t main_pthread_id;

win32_threads.c

@@ -648,7 +648,7 @@ GC_API int GC_CALL GC_thread_is_registered(void)
     LOCK();
     me = GC_lookup_thread_inner(thread_id);
     UNLOCK();
-    return me != NULL;
+    return me != NULL && !KNOWN_FINISHED(me);
 }
 
 GC_API void GC_CALL GC_register_altstack(void *stack GC_ATTR_UNUSED,