[Ada] Adapt proof of System.Arith_Double

[Ada] Adapt proof of System.Arith_Double

gcc/ada/

	* libgnat/s-aridou.adb (Double_Divide): Adjust proof of lemma
	Prove_Signs, call lemma for commutation of Big and
	multiplication.
	(Multiply_With_Ovflo_Check): Adjust postcondition of
	Prove_Pos_Int.
	(Scaled_Divide): Explicit commutation in the proof of lemma
	Prove_Multiplication, add new lemma Prove_Shift_Progress for
	congruence property that is not proved in a larger context, add
	assertions at the end of the loop to state loop invariant
	properties.