[Ada] Proof of System.Val_Uns at gold level

[Ada] Proof of System.Val_Uns at gold level

gcc/ada/

	* libgnat/a-tiinau.ads: Use a procedure for the Scan parameter
	instead of a function with side-effects.
	* libgnat/a-tiinau.adb: Idem.
	* libgnat/a-wtinau.ads: Idem.
	* libgnat/a-wtinau.adb: Idem.
	* libgnat/a-ztinau.ads: Idem.
	* libgnat/a-ztinau.adb: Idem.
	* libgnat/s-valint.ads: Change the function with side-effects
	Scan_Integer into a procedure
	* libgnat/s-vallli.ads: Idem.
	* libgnat/s-valllli.ads: Idem.
	* libgnat/s-vallllu.ads: Add SPARK_Mode and pragma to ignore
	assertions in instance.
	* libgnat/s-valllu.ads: Idem.
	* libgnat/s-valuns.ads: Idem.
	* libgnat/s-valuei.ads: Use a procedure for the
	Scan_Raw_Unsigned parameter instead of a function with
	side-effects and change the function with side-effects
	Scan_Integer into a procedure.
	* libgnat/s-valuei.adb: Idem.
	* libgnat/s-valuti.ads: Introduce a ghost function that scans an
	exponent and complete the postcondition of Scan_Exponent to also
	describe the value of Ptr after the call. Fix the postcondition
	of Scan_Underscore. Simplify the definition of
	Scan_Natural_Ghost.
	* libgnat/s-valuti.adb: Idem.
	* libgnat/s-valboo.ads, libgnat/s-valboo.adb: Update calls to
	First_Non_Space_Ghost.
	* libgnat/s-valueu.ads: Add functional contracts.
	* libgnat/s-valueu.adb: Idem.