[Ada] Avoid artificial underflow in System.Val_Real

gcc/ada/

	* libgnat/s-valrea.adb (Maxexp32): New constant array.
	(Maxexp64): Likewise.
	(Maxexp80): Likewise.
	(Integer_to_Real): New local constants Maxexp and B.
	When the exponent is too negative, do the divison in two steps.