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/common/util/BUILD b/aos/common/util/BUILD
index 8fcd2fe..0f2ca73 100644
--- a/aos/common/util/BUILD
+++ b/aos/common/util/BUILD
@@ -90,6 +90,9 @@
     hdrs = [
         "string_to_num.h",
     ],
+    compatible_with = [
+        "//tools:armhf-debian",
+    ],
 )
 
 cc_test(
@@ -172,6 +175,9 @@
     hdrs = [
         "options.h",
     ],
+    compatible_with = [
+        "//tools:armhf-debian",
+    ],
 )
 
 cc_test(
@@ -190,6 +196,9 @@
     hdrs = [
         "compiler_memory_barrier.h",
     ],
+    compatible_with = [
+        "//tools:armhf-debian",
+    ],
 )
 
 cc_library(