[Ada] Use non-internal representation for access subprograms if UC to Address

[Ada] Use non-internal representation for access subprograms if UC to Address

gcc/ada/

	* libgnat/g-spipat.ads (Boolean_Func, Natural_Func,
	VString_Func): Mark as Favor_Top_Level.
	* sem_ch13.adb (Validate_Unchecked_Conversion): Avoid using
	internal representation if Unchecked_Conversion between
	an access to subprogram and System.Address within the same unit.