Add regression test for atomics being Send + Sync

Thanks to Tamir Duberstein for the implementation suggestion.

src/test/run-pass/sync-send-atomics.rs

@@ -0,0 +1,22 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// pretty-expanded FIXME #23616
+
+use std::sync::atomic::*;
+
+trait SendSync: Send + Sync {}
+
+impl SendSync for AtomicBool {}
+impl SendSync for AtomicIsize {}
+impl SendSync for AtomicUsize {}
+impl<T> SendSync for AtomicPtr<T> {}
+
+fn main() {}