The arch-specific SSA backends now no longer use gc.Sym either.

Passes toolstash -cmp.

Change-Id: Ic13b934b92a1b89b4b79c6c4796ab0a137608163
Reviewed-on: https://go-review.googlesource.com/36416
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>