[Ada] Fix computation of Prec/Succ of zero without denormals

[Ada] Fix computation of Prec/Succ of zero without denormals

gcc/ada/

	* libgnat/s-fatgen.adb: Add use clause for Interfaces.Unsigned_16
	and Interfaces.Unsigned_32.
	(Small16): New constant.
	(Small32): Likewise.
	(Small64): Likewise.
	(Small80): Likewise.
	(Pred): Declare a local overlay for Small and return it negated
	for zero if the type does not support denormalized numbers.
	(Succ): Likewise, but return it directly.