add arm support to travis. native aarch64 and emulated arm32 on x64

Author: kristjanvalur

.travis.yml

@@ -7,4 +7,11 @@ jobs:
     - name: x86_gcc
       before_install: sudo apt install -y gcc-multilib g++-multilib
       env: PLATFORMFLAGS=-m32
-    - arch: arm64
+    - arch: arm64
+    - name: arm32_xcompile
+      before_install:
+        - sudo apt install -y gcc-arm-linux-gnueabi g++-aarch64-linux-gnu
+        - sudo apt install -y qemu-user
+      env: 
+        - PLATFORM_PREFIX=aarch64-linux-gnu-
+        - EMULATOR=qemu-armhf