Simple change to allow plugins for linux on s390x

Change-Id: I5c262ab81aac10d1dcb03381a48e5b9694b7a87a
Reviewed-on: https://go-review.googlesource.com/37451
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>