ada: Implement fast modulo reduction for nonbinary modular multiplication

This implements modulo reduction for nonbinary modular multiplication with
small moduli by means of the standard division-free algorithm also used in
the optimizer, but with fewer constraints and therefore better results.

For the sake of consistency, it is also used for the 'Mod attribute of the
same modular types and, more generally, for the Mod (and Rem) operators of
unsigned types if the second operand is static and not a power of two.

gcc/ada/

	* gcc-interface/gigi.h (fast_modulo_reduction): Declare.
	* gcc-interface/trans.cc (gnat_to_gnu) <N_Op_Mod>: In the unsigned
	case, call fast_modulo_reduction for {FLOOR,TRUNC}_MOD_EXPR if the
	RHS is a constant and not a power of two, and the precision is not
	larger than the word size.
	* gcc-interface/utils2.cc: Include expmed.h.
	(fast_modulo_reduction): New function.
	(nonbinary_modular_operation): Call fast_modulo_reduction for the
	multiplication if the precision is not larger than the word size.

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