Handles spurious wake-ups in pthread_join()

Removed 'join_count' from pthread_internal_t and switched to using the flag
PTHREAD_ATTR_FLAG_JOINED to indicate if a thread is being joined. Combined with
a switch to a while loop in pthread_join, this fixes spurious wake-ups but
prevents a thread from being joined multiple times. This is fine for
two reasons:

1) The pthread_join specification allows for undefined behavior when multiple
   threads try to join a single thread.

2) There is no thread safe way to allow multiple threads to join a single
   thread with the pthread interface.  The second thread calling pthread_join
   could be pre-empted until the thread is destroyed and its handle reused for
   a different thread.  Therefore multi-join is always an error.

Bug: https://code.google.com/p/android/issues/detail?id=52255
Change-Id: I8b6784d47620ffdcdbfb14524e7402e21d46c5f7

libc/bionic/pthread_create.cpp

@@ -110,7 +110,6 @@ int _init_thread(pthread_internal_t* thread, bool add_to_thread_list) {
   }
 
   pthread_cond_init(&thread->join_cond, NULL);
-  thread->join_count = 0;
   thread->cleanup_stack = NULL;
 
   if (add_to_thread_list) {