std: Stabilize the once_new feature

This function follows the well-established "constructor" pattern and the
initialization constant will likely be deprecated in favor of it once `const_fn`
is stabilized.

Author: alexcrichton

src/libstd/sync/once.rs

@@ -48,7 +48,7 @@ pub const ONCE_INIT: Once = Once::new();
 
 impl Once {
     /// Creates a new `Once` value.
-    #[unstable(feature = "once_new")]
+    #[stable(feature = "once_new", since = "1.2.0")]
     pub const fn new() -> Once {
         Once {
             mutex: StaticMutex::new(),