cmd/buildlet, dashboard: add linux-arm-arm5 buildlet

Updates golang/go#17082
Updates golang/go#19662

Change-Id: I2b8294f9a8d9144033f8a3479a64186b0f5143a2
Reviewed-on: https://go-review.googlesource.com/38453
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed