Add s390x build workflow (#3375)

Author: uweigand

.github/workflows/main.yml

@@ -440,6 +440,11 @@ jobs:
           target: aarch64-unknown-linux-gnu
           gcc_package: gcc-aarch64-linux-gnu
           gcc: aarch64-linux-gnu-gcc
+        - build: s390x-linux
+          os: ubuntu-latest
+          target: s390x-unknown-linux-gnu
+          gcc_package: gcc-s390x-linux-gnu
+          gcc: s390x-linux-gnu-gcc
     steps:
     - uses: actions/checkout@v2
       with: