cmd/dist: don't compile unneeded GOARCH SSA rewrite rules during bootstrap

Speeds up build (the bootstrap phase) by ~6 seconds.

Bootstrap goes from ~18 seconds to ~12 seconds.

Change-Id: I7e2ec8f5fc668bf6168d90098eaf70390b16e479
Reviewed-on: https://go-review.googlesource.com/40503


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>