[Ada] Fix floating-point exponentiation with Integer'First exponent

gcc/ada/

	* urealp.adb (Scale): Change first paramter to Uint and adjust.
	(Equivalent_Decimal_Exponent): Pass U.Den directly to Scale.
	* libgnat/s-exponr.adb (Negative): Rename to...
	(Safe_Negative): ...this and change its lower bound.
	(Exponr): Adjust to above renaming and deal with Integer'First.