[Ada] Make System.Generic_Bignums more flexible

[Ada] Make System.Generic_Bignums more flexible

gcc/ada/

	* Makefile.rtl (GNATRTL_NONTASKING_OBJS): Add s-shabig.o.
	* libgnat/s-shabig.ads: New file to share definitions.
	* libgnat/s-genbig.ads, libgnat/s-genbig.adb: Reorganized to
	make it more generic and flexible in terms of memory allocation
	and data structure returned.
	(To_String): Moved to System.Generic_Bignums to allow sharing
	this code.
	(Big_And, Big_Or, Big_Shift_Left, Big_Shift_Right): New.
	* libgnat/s-bignum.adb, libgnat/s-bignum.ads: Adapt to new
	System.Generic_Bignums spec.
	* libgnat/a-nbnbin.adb: Likewise.
	(To_String): Moved to System.Generic_Bignums to allow sharing
	this code.
	* libgnat/a-nbnbre.adb (Normalize): Fix handling of Num = 0
	leading to an exception.