[Ada] Unrecursify Set_Digit/Set_Image_Unsigned procedure

[Pierre-Marie de Rodat]

This patch remove recursion on Set_Digit and Set_Image_Unsigned
procedure.

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

gcc/ada/

	* libgnat/s-imagei.adb
	(Set_Digits): Rewrite the procedure to remove recursion.
	(Image_Integer, Set_Image_Integer): Update assertions and remove
	redundant ones.
	* libgnat/s-imageu.adb
	(Set_Image_Unsigned): Rewrite the procedure to remove recursion.
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