See RM A.3.6 and AI12-0260.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* libgnat/a-zchhan.ads, libgnat/a-zchhan.adb
	(Character_Set_Version, Is_Basic, To_Basic): New.
	* libgnat/a-zchuni.ads, libgnat/a-zchuni.adb (Is_Basic,
	To_Basic): New.