ada: Preserve capability validity in address arithmetic

ada: Preserve capability validity in address arithmetic

On CHERI targets where System.Address is a capability, arithmetic on
addresses should avoid converting to integers and instead use the
operations defined in System.Storage_Elements to perform the arithmetic
directly on the System.Address object. This preserves the capability's
validity throughout the calculation, ensuring that the resulting capability
can be dereferenced.

gcc/ada/

	* libgnat/s-carsi8.adb: Use operations from
	System.Storage_Elements for address arithmetic.
	* libgnat/s-carun8.adb: Likewise
	* libgnat/s-casi128.adb: Likewise
	* libgnat/s-casi16.adb: Likewise
	* libgnat/s-casi32.adb: Likewise
	* libgnat/s-casi64.adb: Likewise
	* libgnat/s-caun128.adb: Likewise
	* libgnat/s-caun16.adb: Likewise
	* libgnat/s-caun32.adb: Likewise
	* libgnat/s-caun64.adb: Likewise
	* libgnat/s-geveop.adb: Likewise