[Ada] Do not use hash function for enumeration Value with trampolines

gcc/ada/

	* exp_imgv.adb: Add with/use clauses for Targparm.
	(Build_Enumeration_Image_Tables): Set type of Threshold to Nat and
	initialize it to Nat'Last if the type is local and the target does
	not support descriptors.  Adjust Threshold_For_Size similarly.
	(Expand_Value_Attribute): Minor tweaks.