Enable arm64 build

Author: kristjanvalur

.travis.yml

@@ -6,4 +6,5 @@ jobs:
     - name: x86_64_gcc
     - name: x86_gcc
       before_install: sudo apt install -y gcc-multilib g++-multilib
-      env: PLATFORMFLAGS=-m32
+      env: PLATFORMFLAGS=-m32
+    - arch: arm64