[Ada] Add ghost code to facilitate proof with SPARK

[Ada] Add ghost code to facilitate proof with SPARK

Proof of generic units for Long_Long_Long_Unsigned instantiations is
harder for provers, as they have to deal with larger values. Add ghost
code to make the proof easier.

gcc/ada/

	* libgnat/s-imageu.adb (Set_Image_Unsigned): Add lemma.
	* libgnat/s-valueu.adb (Scan_Raw_Unsigned): Add assertion.