[dev.link] cmd/internal/obj: change writer to use new-style accessors

Introduce field setters and use them on the writer side. Now we
are able to eliminate the old-style types.

Change-Id: I650d837328dc02f9be839d16a31812be86721b91
Reviewed-on: https://go-review.googlesource.com/c/go/+/227640


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>