[Ada] Fix internal error on fixed-point divide, multiply and scaling

[Ada] Fix internal error on fixed-point divide, multiply and scaling

gcc/ada/

	* exp_fixd.adb (Get_Size_For_Value): New function returning a size
	suitable for a non-negative integer value.
	(Get_Type_For_Size): New function returning a standard type suitable
	for a size.
	(Build_Divide): Call both functions to compute the result type, but
	make sure to pass a non-negative value to the first.
	(Build_Multiply): Likewise.
	(Do_Multiply_Fixed_Universal): Minor consistency tweak.
	(Integer_Literal): Call both functions to compute the type.