[Ada] Proof of System.Val_Int at gold level

[Ada] Proof of System.Val_Int at gold level

gcc/ada/

	* libgnat/s-valint.ads: Add SPARK_Mode and pragma to ignore
	assertions in instance and add additional ghost parameters to
	the instance of Value_I.
	* libgnat/s-vallli.ads: Idem.
	* libgnat/s-valllli.ads: Idem.
	* libgnat/s-valuei.ads, libgnat/s-valuei.adb: New generic
	parameters for ghost functions from System.Valueu. Add
	functional contracts.