[Ada] Time_IO.Value: Allow subseconds and time zones together

[Ada] Time_IO.Value: Allow subseconds and time zones together

gcc/ada/

	* libgnat/g-catiio.ads: Change the regular expression that
	documents the allowed format to match what ISO-8601 allows.
	* libgnat/g-catiio.adb (Scan_Subsecond): Rewrite so it doesn't
	assume the subsecond comes last.
	(Parse_ISO_8601): Parse an optional subsecond, followed by an
	optional time zone, rather than making these alternatives to
	each other.