bsps/x86_64: Move SPDX line to top of file

The RTEMS Software Engineering Guide specifies that the SPDX license
annotation shouldbe the first line of the file and not part of the
copyright/license comment block.

bsps/x86_64/amd64/include/multiboot2impl.h

@@ -1,6 +1,6 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+
 /*
- * SPDX-License-Identifier: BSD-2-Clause
- *
  * Copyright (C) 2023 Karel Gardas
  *
  * Redistribution and use in source and binary forms, with or without