Run tests on userspace-emulated PowerPC

This has the downside of not supporting running tests on Valgrind, see
the discussion in issue #15. However, this does test quite a lot
already.

This uses the docker container defined in
https://github.com/thomwiggers/debian-unstable-powerpc/.

Co-authored-by: Thom Wiggers <thom@thomwiggers.nl>

.travis.yml

@@ -26,6 +26,15 @@ matrix:
             - python3
             - python3-yaml
             - valgrind
+    - name: "Run tests on qemu-ppc"
+      os: linux
+      services: docker
+      env:
+        - MAKETARGET="run-functest-all run-testvectors-all run-sanitizer-all run-symbol-namespace-all"
+      script:  # TODO: should we do this in a Dockerfile instead?
+        - docker run --rm --privileged multiarch/qemu-user-static:register --reset
+        - docker run --rm -v `pwd`:`pwd` -w `pwd` "twiggers/debian-sid-powerpc" /bin/bash -c "uname -a &&
+          make ${MAKETARGET}"
     - name: "MacOS + Clang"
       os: osx
       osx_image: xcode10.1