Relax orderings of Serialize impl for atomic types to match the lates…

…t stable

serde/src/ser/impls.rs

@@ -916,7 +916,7 @@ macro_rules! atomic_impl {
                     S: Serializer,
                 {
                     // Matches the atomic ordering used in libcore for the Debug impl
-                    self.load(Ordering::SeqCst).serialize(serializer)
+                    self.load(Ordering::Relaxed).serialize(serializer)
                 }
             }
         )*