travis: add arm64 LTS targets

Let's start with arm64 for now, then add more arch's later, if people
care about them.

No doubt these need some patches backported.  Let's see what's broken!

Let's keep them non-cron to see what's broken presubmit.

.travis.yml

@@ -25,6 +25,16 @@ matrix:
     - name: "ARCH=x86_64 REPO=linux-next"
       env: ARCH=x86_64 REPO=linux-next
       if: type = cron
+    # stable
+    # TODO: make cron only
+    - name: "ARCH=arm64 REPO=4.18"
+      env: ARCH=arm64 REPO="4.18"
+    - name: "ARCH=arm64 REPO=4.14"
+      env: ARCH=arm64 REPO="4.14"
+    - name: "ARCH=arm64 REPO=4.9"
+      env: ARCH=arm64 REPO="4.9"
+    - name: "ARCH=arm64 REPO=4.4"
+      env: ARCH=arm64 REPO="4.4"
 compiler: gcc
 os: linux
 dist: trusty