[dev.link] cmd/link: small cleanup of loader ocache

[dev.link] cmd/link: small cleanup of loader ocache

Remove references to the loader.Loader's "ocache" field, which is no
longer being used the way it was. Add in a new field for object index
to the oReader type.

Change-Id: Icae6c54dbbdbbfa63ab9de3e4dc09e10abd57ee3
Reviewed-on: https://go-review.googlesource.com/c/go/+/222063
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>