[dev.link] cmd/internal/goobj2, cmd/link: add and use new Sym accessors

Along the line with Reloc and Aux, use new-style accessors for
the Sym type. They avoid reading unnecessary fields, and also
look nicer (to me).

Change-Id: Ie37c5149a6edb2184724b3dfa26952015e74c085
Reviewed-on: https://go-review.googlesource.com/c/go/+/226798


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