ax-gdb: Use ax->gdbarch instead of exp->gdbarch, remove unused parameters

ax-gdb: Use ax->gdbarch instead of exp->gdbarch, remove unused parameters

In many ax generation functions, the "expression *exp" parameter is only
used to access the gdbarch.  The same value can be found in the
"agent_expr *ax" parameter, which needs to be passed in any case.  By
using ax->gdbarch instead of exp->gdbarch, we can avoid passing exp in
many of these functions.

gdb/ChangeLog:

	* ax-gdb.c (gen_usual_unary): Remove exp parameter, get gdbarch
	from ax, update calls.
	(gen_usual_arithmetic): Likewise.
	(gen_integral_promotions): Likewise.
	(gen_bitfield_ref): Likewise.
	(gen_primitive_field): Likewise.
	(gen_struct_ref_recursive): Likewise.
	(gen_struct_ref): Likewise.
	(gen_maybe_namespace_elt): Likewise.
	(gen_struct_elt_for_reference): Likewise.
	(gen_namespace_elt): Likewise.
	(gen_aggregate_elt_ref): Likewise.
	(gen_expr): Get gdbarch from ax, update calls.
	(gen_expr_binop_rest): Likewise.