[Ada] Factor out machine rounding operations

[Ada] Factor out machine rounding operations

gcc/ada/

	* sem_eval.ads (Machine_Number): New inline function.
	* sem_eval.adb (Machine_Number): New function body implementing
	the machine rounding operation specified by RM 4.9(38/2).
	(Check_Non_Static_Context): Call Machine_Number and set the
	Is_Machine_Number flag consistently on the resulting node.
	* sem_attr.adb (Eval_Attribute) <Attribute_Machine>: Likewise.
	* checks.adb (Apply_Float_Conversion_Check): Call Machine_Number.
	(Round_Machine): Likewise.