[Ada] Correctly round Value attribute for floating point in more cases

This provides correct rounding in the IEEE 754 sense for the Value attribute
of floating-point types in more cases, by switching from tables of powers of
10 to tables of powers of 5 for precomputed values, thus making it possible
to use a single divide for denormals and normalized numbers just above them.

Although this significantly increases the size of the tables, object files
for them are still quite small (1, 2 and 4 KB respectively on x86-64).

gcc/ada/

	* libgnat/s-powflt.ads (Powfive): New constant array.
	* libgnat/s-powlfl.ads (Powfive): Likewise.
	(Powfive_100): New constant.
	(Powfive_200): Likewise.
	(Powfive_300): Likewise.
	* libgnat/s-powllf.ads (Powfive): New constant array.
	(Powfive_100): New constant.
	(Powfive_200): Likewise.
	(Powfive_300): Likewise.
	* libgnat/s-valflt.ads (Impl): Replace Powten with Powfive and pass
	Null_Address for the address of large constants.
	* libgnat/s-vallfl.ads (Impl): Replace Powten with Powfive and pass
	the address of large constants.
	* libgnat/s-valllf.ads (Impl): Likewise.
	* libgnat/s-valrea.ads (System.Val_Real): Replace Powten_Address
	with Powfive_Address and add Powfive_{1,2,3}00_Address parameters.
	* libgnat/s-valrea.adb (Is_Large_Type): New boolean constant.
	(Is_Very_Large_Type): Likewise.
	(Maxexp32): Change value of 10 to that of 5.
	(Maxexp64): Likewise.
	(Maxexp80): Likewise.
	(Integer_to_Real): Use a combination of tables of powers of 5 and
	scaling if the base is 10.
	(Large_Powten): Rename into...
	(Large_Powfive): ...this.  Add support for large constants.
	(Large_Powfive): New overloaded function for very large exponents.

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