dashboard: restore darwin-386 buidler

Fixes golang/go#17009

Change-Id: Ia9008bcb7b47b7671c158c1a00afc4e765805e4e
Reviewed-on: https://go-review.googlesource.com/41617
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: bradfitz

dashboard/builders.go

@@ -1007,6 +1007,12 @@ func init() {
 		numTestHelpers:    2,
 		numTryTestHelpers: 3,
 	})
+	addBuilder(BuildConfig{
+		Name:      "darwin-386-10_11",
+		HostType:  "host-darwin-10_11",
+		MaxAtOnce: 1,
+		env:       []string{"GOARCH=386", "GOHOSTARCH=386"},
+	})
 	addBuilder(BuildConfig{
 		Name:     "darwin-amd64-10_12",
 		HostType: "host-darwin-10_12",