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/third_party/eigen/BUILD b/third_party/eigen/BUILD
index 4a33fd9..8fe7fa6 100644
--- a/third_party/eigen/BUILD
+++ b/third_party/eigen/BUILD
@@ -1,22 +1,28 @@
-licenses(['notice'])
+licenses(["notice"])
 
 load("//tools:environments.bzl", "mcu_cpus")
 
 cc_library(
-  name = 'eigen',
-  visibility = ['//visibility:public'],
-  srcs = glob(['Eigen/src/**/*.h']),
-  includes = ['.'],
-  hdrs = glob(['Eigen/*'], exclude=[
-    # Stuff that we don't have the dependencies for.
-    'Eigen/CholmodSupport',
-    'Eigen/MetisSupport',
-    'Eigen/PaStiXSupport',
-    'Eigen/PardisoSupport',
-    'Eigen/SPQRSupport',
-    'Eigen/SuperLUSupport',
-    'Eigen/UmfPackSupport',
-  ]) + ['unsupported/Eigen/MatrixFunctions'] +
-  glob(['unsupported/Eigen/src/MatrixFunctions/*.h']),
-  compatible_with = mcu_cpus,
+    name = "eigen",
+    srcs = glob(["Eigen/src/**/*.h"]),
+    hdrs = glob(
+        ["Eigen/*"],
+        exclude = [
+            # Stuff that we don't have the dependencies for.
+            "Eigen/CholmodSupport",
+            "Eigen/MetisSupport",
+            "Eigen/PaStiXSupport",
+            "Eigen/PardisoSupport",
+            "Eigen/SPQRSupport",
+            "Eigen/SuperLUSupport",
+            "Eigen/UmfPackSupport",
+        ],
+    ) + ["unsupported/Eigen/MatrixFunctions"] + glob([
+        "unsupported/Eigen/src/MatrixFunctions/*.h",
+    ]),
+    compatible_with = mcu_cpus + [
+        "//tools:armhf-debian",
+    ],
+    includes = ["."],
+    visibility = ["//visibility:public"],
 )