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/aos/BUILD b/aos/BUILD
index 2e73c51..8aa274c 100644
--- a/aos/BUILD
+++ b/aos/BUILD
@@ -45,6 +45,9 @@
     hdrs = [
         "once.h",
     ],
+    compatible_with = [
+        "//tools:armhf-debian",
+    ],
     visibility = ["//visibility:public"],
     deps = [
         "//aos/common:gtest_prod",