Merge pull request #1364 from Unity-Technologies/unity-master-fix-1295072

After adding the concept of a pending threads join list boehm was not…

Author: UnityAlex

mono/metadata/boehm-gc.c

@@ -567,8 +567,7 @@ mono_gc_thread_detach_with_lock (MonoThreadInfo *p)
 
 	tid = mono_thread_info_get_tid (p);
 
-	if (p->runtime_thread)
-		mono_threads_add_joinable_thread ((gpointer)tid);
+	mono_threads_add_joinable_runtime_thread(p);
 
 	mono_handle_stack_free (p->handle_stack);
 	p->handle_stack = NULL;