[Ada] Small tweaks to couple of Value routines

[Pierre-Marie de Rodat]

This fixes a minor oversight in Integer_To_Fixed and restricts the
previous change to System.Value_R.Scan_Raw_Real for the sake of
backward compatibility with earlier conversions of the compiler.

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

gcc/ada/

	* libgnat/s-valuef.adb (Integer_To_Fixed): Take into account the
	extra digit when scaling up the input.
	* libgnat/s-valuer.adb (Scan_Decimal_Digits): Restrict previous
	change to fixed-point types.
	(Scan_Integral_Digits): Likewise.
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