runtime: generate dummy duffcopy

Although duffcopy is not used on PPC64, duff_ppc64x.s and
mkduff.go don't match. Make it so.

Fixes #38188.

Change-Id: Ic6c08e335795ea407880efd449f4229696af7744
Reviewed-on: https://go-review.googlesource.com/c/go/+/226719


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>