dashboard: test sub-repositories on s390x

Fixes golang/go#26131

Change-Id: I07e84c255a158b843168612cc3dad7688343ffb7
Reviewed-on: https://go-review.googlesource.com/127375
Reviewed-by: Michael Munday <mike.munday@ibm.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

Author: bradfitz

dashboard/builders.go

@@ -762,6 +762,7 @@ func (c *BuildConfig) BuildSubrepos() bool {
 		"freebsd-386-10_3", "freebsd-amd64-10_3",
 		"freebsd-386-11_1", "freebsd-amd64-11_1",
 		"linux-386", "linux-amd64", "linux-amd64-nocgo",
+		"linux-s390x-ibm",
 		"openbsd-386-60", "openbsd-amd64-60",
 		"openbsd-386-62", "openbsd-amd64-62",
 		"netbsd-amd64-8_0",