xen/x86: Replace GPL v2.0 license boilerplate with an SPDX tag in *.h

xen/x86: Replace GPL v2.0 license boilerplate with an SPDX tag in *.h

It is easier to understand the license of a file when using SPDX.

This is replacing the below pattern with the SPDX tag GPL-2.0-only
in xen/arch/x86/*.h:

 * This program is free software; you can redistribute it and/or modify it
 * under the terms and conditions of the GNU General Public License,
 * version 2, as published by the Free Software Foundation.
 *
 * This program is distributed in the hope it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along with
 * this program; If not, see <http://www.gnu.org/licenses/>.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>