Add stability attributes

Author: Stjepan Glavina

src/libstd/thread/mod.rs

@@ -1276,7 +1276,9 @@ impl<T> JoinInner<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct JoinHandle<T>(JoinInner<T>);
 
+#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
 unsafe impl<T> Send for JoinHandle<T> {}
+#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
 unsafe impl<T> Sync for JoinHandle<T> {}
 
 impl<T> JoinHandle<T> {