[Ada] Time_IO.Image: Allow printing the time zone

[Ada] Time_IO.Image: Allow printing the time zone

gcc/ada/

	* libgnat/g-catiio.ads: Document newly supported format.  Add
	ISO_Time constant, for convenience.
	* libgnat/g-catiio.adb (Image_Helper): New helper function to do
	all the formatting work, called by the two exported Image
	functions.  Add support for "%:::z" here. Add a Time_Zone
	parameter used by the "%:::z" processing. This parameter is not
	used for the actual time zone computations; local time is always
	used for that, for ease of implementation reasons.  It would
	make sense to use Append throughout this function, but that's a
	cleanup for another day.
	(Image): Modify these to pass the local time zone, or the
	specified time zone, as appropriate.