The arch backends no longer depend on gc.Node.

Passes toolstash-check -all.

Change-Id: Ic7e49ae0a3ed155a2761c25e17cc341b46333fb4
Reviewed-on: https://go-review.googlesource.com/41196
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>