[dev.link] cmd/link: use new-style Aux accessors in the loader

And remove the old-style one.

Change-Id: Ia6c84448108dd1da68451df2221c7c1005c45a0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/226364


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>