cmd/coordinator: cross-compile ARM build on Kubernetes

Saves 4.5 minutes or so by using fast x86 machines to build the ARM
build instead of running make.bash on Scaleway ARM machines.

We still run the tests on ARM, and have a separate builder only
running make.bash on ARM (see prior golang.org/cl/29670)

Fixes golang/go#17105
Updates golang/go#17104

Change-Id: I1cb7b0e5b1cc8b644195f262328884ed3aff120a
Reviewed-on: https://go-review.googlesource.com/29677
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index e648920..57379a9 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -28,6 +28,13 @@
 		buildletURLTmpl: "http://storage.googleapis.com/$BUCKET/buildlet.linux-amd64",
 		env:             []string{"GOROOT_BOOTSTRAP=/go1.4"},
 	},
+	"host-linux-armhf-cross": &HostConfig{
+		Notes:           "Kubernetes container on GKE built from env/crosscompile/linux-armhf-jessie",
+		KubeImage:       "linux-armhf-jessie:latest",
+		buildletURLTmpl: "http://storage.googleapis.com/$BUCKET/buildlet.linux-amd64",
+		env:             []string{"GOROOT_BOOTSTRAP=/go1.4"},
+	},
+
 	"host-nacl-kube": &HostConfig{
 		Notes:           "Kubernetes container on GKE.",
 		KubeImage:       "linux-x86-nacl:latest",