This AI updates among other things some declarations in big number
packages.

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

gcc/ada/

	* libgnat/a-nbnbin.ads (From_Universal_Image): New.
	(Big_Integer): Update definition.
	* libgnat/a-nbnbre.ads, libgnat/a-nbnbre.adb
	(From_Universal_Image): New.
	(From_String): Remove alternate body, replaced by
	From_Universal_Image.
	(Big_Real): Update definition.