dashboard: add s390x cross-compile-only builder and release config

Also modifies cmd/release to wire it up.

Updates golang/go#16443

Change-Id: Ie80a224957f6f662d0aec0fd083e2a57e30436c4
Reviewed-on: https://go-review.googlesource.com/28172
Reviewed-by: Jessica Frazelle <me@jessfraz.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index b76f26b..a18e953 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -624,6 +624,22 @@
 		env:            []string{"GOROOT_BOOTSTRAP=/var/buildlet/go-linux-s390x-bootstrap"},
 	})
 	addBuilder(BuildConfig{
+		Name:            "linux-s390x-crosscompile",
+		Notes:           "s390x cross-compile builder for releases; doesn't run tests",
+		KubeImage:       "linux-s390x-stretch:latest",
+		CompileOnly:     true,
+		TryOnly:         true, // but not in trybot set for now
+		buildletURLTmpl: "https://storage.googleapis.com/$BUCKET/buildlet.linux-amd64",
+		env: []string{
+			"GOROOT_BOOTSTRAP=/go1.4",
+			"CGO_ENABLED=1",
+			"GOARCH=s390x",
+			"GOHOSTARCH=amd64",
+			"CC_FOR_TARGET=s390x-linux-gnu-gcc",
+		},
+	})
+
+	addBuilder(BuildConfig{
 		Name:           "solaris-amd64-oraclejtsylve",
 		Notes:          "temporary test builder run by jtsylve",
 		IsReverse:      true,