u_queue: add a futex-based implementation of fences

Fences are now 4 bytes instead of 96 bytes (on my 64-bit system).

Signaling a fence is a single atomic operation in the fast case plus a
syscall in the slow case.

Testing if a fence is signaled is the same as before (a simple comparison),
but waiting on a fence is now no more expensive than just testing it in
the fast (already signaled) case.

v2:
- style fixes
- use p_atomic_xxx macros with the right barriers

Acked-by: Marek Olšák <marek.olsak@amd.com>

src/util/u_queue.c

@@ -89,6 +89,7 @@ remove_from_atexit_list(struct util_queue *queue)
  * util_queue_fence
  */
 
+#ifdef UTIL_QUEUE_FENCE_STANDARD
 void
 util_queue_fence_signal(struct util_queue_fence *fence)
 {
@@ -136,6 +137,7 @@ util_queue_fence_destroy(struct util_queue_fence *fence)
    cnd_destroy(&fence->cond);
    mtx_destroy(&fence->mutex);
 }
+#endif
 
 /****************************************************************************
  * util_queue implementation