Add "common-defs.h" include to files in arch/ subdir not yet including it.

This fixes a build breakage on FreeBSD hosts.

gdb/ChangeLog:

	* arch/aarch64.c: Include "common-defs.h".
	* arch/amd64.c: Likewise.
	* arch/i386.c: Likewise.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2018-02-21  John Baldwin  <jhb@FreeBSD.org>
+
+	* arch/aarch64.c: Include "common-defs.h".
+	* arch/amd64.c: Likewise.
+	* arch/i386.c: Likewise.
+
 2018-02-21  Tom Tromey  <tom@tromey.com>
 
 	* value.h: (extract_field_op): Update.

gdb/arch/aarch64.c

@@ -15,7 +15,7 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
-
+#include "common-defs.h"
 #include "aarch64.h"
 #include <stdlib.h>
 

gdb/arch/amd64.c

@@ -15,6 +15,7 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+#include "common-defs.h"
 #include "amd64.h"
 #include "x86-xstate.h"
 #include <stdlib.h>

gdb/arch/i386.c

@@ -15,6 +15,7 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+#include "common-defs.h"
 #include "i386.h"
 #include "tdesc.h"
 #include "x86-xstate.h"