distro/rhel8: introduce s390x support

This patch introduces support for s390x architecture. It has some
specific requirements like using kernel-cmdline stage before the RPM
stage because RPM scriptlets require these values to be present in the
system already. It also uses zipl stage and bootloader as opposed to all
the other architectures.

test/cases/rhel_8-s390x-tar-boot.json

@@ -1553,6 +1553,13 @@
         "runner": "org.osbuild.rhel82"
       },
       "stages": [
+        {
+          "name": "org.osbuild.kernel-cmdline",
+          "options": {
+            "root_fs_uuid": "0bd700f8-090f-4556-b797-b340297ea1bd",
+            "kernel_opts": "net.ifnames=0 crashkernel=auto"
+          }
+        },
         {
           "name": "org.osbuild.rpm",
           "options": {
@@ -2196,6 +2203,10 @@
             }
           }
         },
+        {
+          "name": "org.osbuild.zipl",
+          "options": {}
+        },
         {
           "name": "org.osbuild.selinux",
           "options": {
@@ -6263,4 +6274,4 @@
       "1": "sha256:3d65a2da0ab6a7d04a62c84ff852eccb6641e864f6270a14fbc7a70cdc60ad76"
     }
   }
-}
+}