[arm] Fix cross-native builds

[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