replace legacy copyright annotations in submodules

Author: pietroalbini

example/alloc_system.rs

@@ -1,12 +1,6 @@
-// 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.
+// SPDX-License-Identifier: MIT OR Apache-2.0
+// SPDX-FileCopyrightText: The Rust Project Developers (see https://thanks.rust-lang.org)
+
 #![no_std]
 
 pub struct System;