Add SPDX license identifier if not present already in source file

Fixes: ethereum#9220

Author: sacgrover

libevmasm/Assembly.cpp

@@ -14,6 +14,7 @@
 	You should have received a copy of the GNU General Public License
 	along with solidity.  If not, see <http://www.gnu.org/licenses/>.
 */
+// SPDX-License-Identifier: GPL-3.0
 /** @file Assembly.cpp
  * @author Gav Wood <i@gavwood.com>
  * @date 2014

libevmasm/Assembly.h

@@ -14,6 +14,7 @@
 	You should have received a copy of the GNU General Public License
 	along with solidity.  If not, see <http://www.gnu.org/licenses/>.
 */
+// SPDX-License-Identifier: GPL-3.0
 
 #pragma once