re-add support for s390x

In #315 we added support for s390x, but this was unintentionally removed
in v0.10.0 due to the makefile modules changes. This PR re-adds support
for s390x.

It depended on a new base image being built for s390x, which wasn't
previously created.

Signed-off-by: Ashley Davis <[email protected]>

Author: SgtCoDFish

make/00_mod.mk

@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+oci_platforms := linux/amd64,linux/arm/v7,linux/arm64,linux/ppc64le,linux/s390x
+
 include make/00_debian_version.mk
 
 repo_name := github.com/cert-manager/trust-manager