[Ada] Small cleanup in body of System.Value_R

[Ada] Small cleanup in body of System.Value_R

This is mostly stylistic but also adds a couple of missing comments.

gcc/ada/

	* libgnat/s-valuer.adb (Scan_Decimal_Digits): Consistently avoid
	initializing local variables.
	(Scan_Integral_Digits): Likewise.
	(Scan_Raw_Real): Likewise and add a couple of comments.