When we switched to SSA R0 was made allocatable and no longer holds
zero on s390x.

Change-Id: I1c752bb02da35462a535492379345fa9f4e12cb0
Reviewed-on: https://go-review.googlesource.com/41079Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>