[s390x] add CI

This commit tests `s390x-unknown-linux-gnu` on CI using `qemu-user`.

Closes #499 .

Author: gnzlbg

.travis.yml

@@ -22,6 +22,7 @@ matrix:
     - env: TARGET=powerpc-unknown-linux-gnu
     - env: TARGET=powerpc64-unknown-linux-gnu
     - env: TARGET=powerpc64le-unknown-linux-gnu
+    - env: TARGET=s390x-unknown-linux-gnu NORUN=1
     - os: osx
       env: TARGET=i686-apple-darwin
       script: ci/run.sh

ci/docker/s390x-unknown-linux-gnu/Dockerfile

@@ -0,0 +1,13 @@
+FROM ubuntu:17.10
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        curl ca-certificates \
+        gcc libc6-dev \
+        gcc-s390x-linux-gnu libc6-dev-s390x-cross \
+        qemu-user \
+        make \
+        file
+
+ENV CARGO_TARGET_S390X_UNKNOWN_LINUX_GNU_LINKER=s390x-linux-gnu-gcc \
+    CARGO_TARGET_S390X_UNKNOWN_LINUX_GNU_RUNNER="qemu-s390x -L /usr/s390x-linux-gnu" \
+    OBJDUMP=s390x-linux-gnu-objdump