Add hb_barrier()

And annotate some more.
diff --git a/src/hb-atomic.hh b/src/hb-atomic.hh
index a480093..366fb32 100644
--- a/src/hb-atomic.hh
+++ b/src/hb-atomic.hh
@@ -218,5 +218,11 @@
   T *v = nullptr;
 };
 
+static inline bool hb_barrier ()
+{
+  _hb_compiler_memory_r_barrier ();
+  return true;
+}
+
 
 #endif /* HB_ATOMIC_HH */