[dev.link] cmd/internal/goobj: use new style accessors

We already move to new style accessors in the linker. This will
allow us to get rid of the read side of old style ones.

Change-Id: Id0c171c5634a5977fe8a6f764cb0d48203993ab7
Reviewed-on: https://go-review.googlesource.com/c/go/+/226799


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