cmd/compile: mark duffzero as using X0, not AX

duffzero was changed to use X0 instead of AX in
CL 14408.  This was missed as part of that change.

Change-Id: I72fb0114cfbc035b83bfaa8631d27e6740da2652
Reviewed-on: https://go-review.googlesource.com/16717


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>