This prevents the generation of the new perfect hash function for the
Value attribute of local enumeration types when the target still uses
trampolines to implement pointers to local subprograms.

The generation of trampolines would be unexpected in these circumstances
and maybe even forbidden by the No_Implicit_Dynamic_Code restriction.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.