[Ada] Avoid artificial underflow in System.Val_Real

[Pierre-Marie de Rodat]

The final computation now needs to be protected against artificial
underflow when the value is very small.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.
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