[Ada] Minor fixes mostly in comments of runtime arithmetic unit

[Ada] Minor fixes mostly in comments of runtime arithmetic unit

Multiple comments in functions Double_Divide and Scaled_Divide were
incorrect. Now fixed.

Also change the expression (if Zhi /= 0 then Ylo * Zhi else 0) to the
simpler equivalent (Ylo * Zhi) in Double_Divide.

Also add a comment explaining why the implementation of Algorithm D for
multiple-precision division from the 2nd Edition of The Art of Computer
Programming does not suffer from two bugs discovered on that version.

There is no impact on execution, hence no test.

2019-09-17  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* libgnat/s-arit64.adb (Double_Divide): Simplify needlessly
	complex computation. Fix comments.
	(Scaled_Divide): Fix comments. Explain why implementation does
	not suffer from bugs in Algorithm D from 2nd Edition of TAOCP.

From-SVN: r275792