[Ada] Prove double precision integer arithmetic unit

[Pierre-Marie de Rodat]

This unit is used to implement the rutime support for fixed-point
operations (conversions, multiplication, division and I/O).  Its
correctness is proved with GNATprove.

Proof is performed with GNATprove options --level=4 --prover=all

Proof requires use of the special Big_Integers_Ghost unit for spec and
proof. Mark also the units analyzed as being in SPARK with aspect
SPARK_Mode.

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

gcc/ada/

	* libgnat/a-nbnbig.ads: Mark the unit as Pure.
	* libgnat/s-aridou.adb: Add contracts and ghost code for proof.
	(Scaled_Divide): Reorder operations and use of temporaries in
	two places to facilitate proof.
	* libgnat/s-aridou.ads: Add full functional contracts.
	* libgnat/s-arit64.adb: Mark in SPARK.
	* libgnat/s-arit64.ads: Add contracts similar to those from
	s-aridou.ads.
	* rtsfind.ads: Document the limitation that runtime units
	loading does not work for private with-clauses.
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