[Ada] Reimplement Ada.Numerics.Big_Numbers.Big_Reals.Fixed_Conversions

[Ada] Reimplement Ada.Numerics.Big_Numbers.Big_Reals.Fixed_Conversions

gcc/ada/

	* libgnat/a-nbnbre.adb (Float_Conversions): Instantiate Conv
	package only once in the body.
	(Fixed_Conversions.Float_Aux): New instance.
	(Fixed_Conversions.Conv_I): Likewise.
	(Fixed_Conversions.Conv_U): Likewise.
	(Fixed_Conversions.LLLI): New subtype.
	(Fixed_Conversions.LLLU): Likewise.
	(Fixed_Conversions.Too_Large): New constant.
	(Fixed_Conversions.To_Big_Real): Reimplement.
	(Fixed_Conversions.From_Big_Real): Likewise.