CL 28978 (6ec993ad) accidentally disabled the test (it would only
run if amd64 AND s390x, whereas it should be amd64 OR s390x).

Change-Id: I23c1ad71724ff55f5808d5896b19b62c8ec5af76
Reviewed-on: https://go-review.googlesource.com/28981Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>