[Ada] Small improvement to System.Value_R.Scan_Raw_Real

[Pierre-Marie de Rodat]

This makes System.Value_R.Scan_Raw_Real round the extra digit it returns
to the caller, so that it is precise to the last 'Aft digits for strings
that are the exact representation of a number, i.e. not already rounded.

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

gcc/ada/

	* libgnat/s-valuer.adb (Scan_Decimal_Digits): Round Extra.
	(Scan_Integral_Digits): Likewise.
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