[dev.link] cmd/oldlink: decouple from goobj2 package

The new object file support in the old linker should not be used.
This is a minimal change that removes stuff from the old linker's
loader package, so that it decouples from the goobj2 package,
allowing the latter to evolve.

Keep the change local in the loader package, so most of the old
linker doesn't need to change. At this point I don't think we
want to make significant changes to the old linker.

Change-Id: I078c4cbb35dc4627c4b82f512a4aceec9b594925
Reviewed-on: https://go-review.googlesource.com/c/go/+/226800


Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>