all: add go.mod Updates golang/go#30228 Change-Id: Idde6e34da043b401ee7429a0297517ec235e3b64 Reviewed-on: https://go-review.googlesource.com/c/arch/+/167137 Run-TryBot: Bryan C. Mills <bcmills@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>