travis: Test across a range of platforms

Travis CI now supports testing on arm64, ppc64le and s390x. It would
be nice to test on them.
Signed-off-by: Daniel Axtens <dja@axtens.net>

.travis.yml

@@ -2,6 +2,12 @@ language:
     - c
     - cpp
 
+arch:
+  - amd64
+  - arm64
+  - ppc64le
+  - s390x
+
 dist: bionic
 
 addons:
@@ -30,4 +36,6 @@ script:
     - ( mkdir build-cmake/bin && cd build-cmake/bin && cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_INSTALL_PREFIX=protobuf-c-bin ../ && make -j2 && make test && make install)
 
 after_success:
-    - cpp-coveralls --build-root . --exclude t/ --exclude /usr/include --exclude protobuf-$PROTOBUF_VERSION --exclude protoc-c
+    - if [ $(uname -m) = "x86_64" ]; then
+        cpp-coveralls --build-root . --exclude t/ --exclude /usr/include --exclude protobuf-$PROTOBUF_VERSION --exclude protoc-c;
+      fi