travis: add arm64 LTS targets

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

Looks like 4.14 is supported, but 4.19, 4.9, and 4.4 need more
backports before they can be enabled.

.travis.yml

@@ -30,6 +30,20 @@ matrix:
     - name: "ARCH=x86_64 REPO=linux-next"
       env: ARCH=x86_64 REPO=linux-next
       if: type = cron
+    # stable
+    # TODO: re-enable once green
+    #- name: "ARCH=arm64 REPO=4.19"
+      #env: ARCH=arm64 REPO=4.19
+      #if: type = cron
+    - name: "ARCH=arm64 REPO=4.14"
+      env: ARCH=arm64 REPO=4.14
+      if: type = cron
+    #- name: "ARCH=arm64 REPO=4.9"
+      #env: ARCH=arm64 REPO=4.9
+      #if: type = cron
+    #- name: "ARCH=arm64 REPO=4.4"
+      #env: ARCH=arm64 REPO=4.4
+      #if: type = cron
 compiler: gcc
 os: linux
 dist: trusty