[Ada] Implement Time_IO.Image with Time_Zone

[Ada] Implement Time_IO.Image with Time_Zone

gcc/ada/

	* libgnat/g-catiio.ads, libgnat/g-catiio.adb (Image): New
	function. It might seem like the local-time Image should be
	implemented in terms of the Time_Zone Image, but that would be
	far harder to implement, so we do it the other way around.