Use the downloaded clang when building for armhf-debian

This makes it work on a barebones Stretch installation.

Also add building for this CPU to the CI script so we know it keeps
working, which means marking everything that's supposed to work
appropriately.

Change-Id: Ic050ce20eae45c6b23e0e42dddb24db3ebc70b84
diff --git a/frc971/control_loops/drivetrain/BUILD b/frc971/control_loops/drivetrain/BUILD
index 90385e0..64d1d9d 100644
--- a/frc971/control_loops/drivetrain/BUILD
+++ b/frc971/control_loops/drivetrain/BUILD
@@ -21,6 +21,9 @@
     srcs = [
         "drivetrain.q",
     ],
+    compatible_with = [
+        "//tools:armhf-debian",
+    ],
     deps = [
         "//aos/common/controls:control_loop_queues",
         "//frc971/control_loops:queues",