gdb: add comment in dwarf_evaluate_loc_desc::push_dwarf_reg_entry_value

gdb: add comment in dwarf_evaluate_loc_desc::push_dwarf_reg_entry_value

Add a comment to clarify why we temporarily override some of the
context's fields, and especially the per_objfile field.  A longer
explanation can be found in this previous commit

    44486dcf19b ("gdb: use caller objfile in dwarf_evaluate_loc_desc::push_dwarf_reg_entry_value")

gdb/ChangeLog:

	* dwarf2/loc.c (class dwarf_evaluate_loc_desc)
	<push_dwarf_reg_entry_value>: Add comment.

Change-Id: I60c6e1062799f729b30a9db78bcb6448783324b4