[arm] Fix cross-native builds

The patch I committed yesterday to remove some generated headers from
the source tree unfortunately has a dependency missing that is only
revealed when doing a cross-native or full Canadian cross build.  The
gen* programs were missing a dependency on one of the generated
headers.

Fixed by adding an explicit dependency rule for GTM_H in the same way
as we do for TM_H.

	* config/arm/t-arm (GTM_H): Add arm-cpu.h.

Checked that this restores cross-native building.

From-SVN: r250015

gcc/ChangeLog

+2017-07-06  Richard Earnshaw  <rearnsha@arm.com>
+
+	* config/arm/t-arm (GTM_H): Add arm-cpu.h.
+
 2017-07-06  Christophe Lyon  <christophe.lyon@linaro.org>
 
 	* doc/sourcebuild.texi (Test Directives, Variants of

gcc/config/arm/t-arm

@@ -19,6 +19,7 @@
 # <http://www.gnu.org/licenses/>.
 
 TM_H += arm-cpu.h
+GTM_H += arm-cpu.h
 
 # All md files - except for arm.md.
 # This list should be kept in alphabetical order and updated whenever an md