ci: travis: add build on ARM64 arch now provided by Travis

Cheap and easy way for us to have basic compile testing on ARM64. Travis
is also offering ppc64le and s390x for builds, but on ppc64le we get a
internal compoler error (yay!) on c++ examples and on s390x we have no
luajit. So, these two have to wait a bit more.

.travis.yml

@@ -77,6 +77,10 @@ jobs:
     - os: linux
       if: type = cron
       env: CI_BUILD_TYPE=codecov
+    - os: linux
+      if: type = cron
+      arch: arm64
+      env: CI_BUILD_TYPE=default
 
 services:
   - docker