Add dwarf2_per_objfile parameter to allocate_piece_closure

This allows removing a dwarf2_per_cu_data::dwarf2_per_objfile reference.

gdb/ChangeLog:

	* dwarf2/loc.c (allocate_piece_closure): Add dwarf2_per_objfile
	parameter.
	(dwarf2_evaluate_loc_desc_full): Update.

Change-Id: Ic4a694a3fc763360a131ee4e3aaf5a5b4735c813