[Ada] Unbounded string overriding control

[Ada] Unbounded string overriding control

gcc/ada/

	* libgnat/a-strunb.adb (Sum, Mul, Saturated_Sum, Saturated_Mul):
	New routines.  Use them when resulting string size more that
	length of the strings in parameters.
	(Unbounded_Slice): Use "- 1" instead of "+ 1" in opposite side
	of condition to avoid overflow.
	* libgnat/a-strunb__shared.adb (Sum, Mul): New routines.
	(Allocate): New routine with 2 parameters.  Use routine above
	when resulting string size more that length of the strings in
	parameters.
	(Aligned_Max_Length): Do not try to align to more than Natural'Last.
	(Unbounded_Slice): Use "- 1" instead of "+ 1" in opposite side
	of condition to avoid overflow.