CI: ubuntu.yml: Add GitHub Actions s390x case

Add the s390x case using GitHub Actions ppc64le/s390x service.
https://github.com/IBM/actionspz

We can run the ppc64le/s390x cases only in the registered upstream repositories.
https://github.com/IBM/actionspz/blob/main/docs/FAQ.md#what-about-forked-repos

The following matrix upstream logic is to skip the ppc64le/s390x in the
downstream (fork) repositories.

```
+        upstream:
+          - ${{ github.repository == 'ruby/ruby' }}
```

Use the "os" list to determine the excluded ppc64le/s390x cases by using the
"exclude" syntax. Because the "exclude" syntax are executed before the
"include" syntax.

Add the ubuntu-24.04-ppc64le as a comment, because the GitHub Actions ppc64le
case has the following test errors and failures.
https://bugs.ruby-lang.org/issues/21534

Author: junaruga

.github/workflows/ubuntu.yml

@@ -24,6 +24,22 @@ jobs:
   make:
     strategy:
       matrix:
+        test_task: [check]
+        configure: ['']
+        arch: ['']
+        os:
+          - ubuntu-24.04
+          - ubuntu-24.04-arm
+          # FIXME Comment out ppc64le due to failing tests on GitHub Actions
+          # ppc64le
+          # https://bugs.ruby-lang.org/issues/21534
+          # - ubuntu-24.04-ppc64le
+          - ubuntu-24.04-s390x
+        # The ppc64le/s390x runners work only in the registered repositories.
+        # They don't work in forked repositories.
+        # https://github.com/IBM/actionspz/blob/main/docs/FAQ.md#what-about-forked-repos
+        upstream:
+          - ${{ github.repository == 'ruby/ruby' }}
         include:
           - test_task: check
             configure: 'cppflags=-DVM_CHECK_MODE'
@@ -36,10 +52,11 @@ jobs:
           - test_task: test-bundler-parallel
             timeout: 50
           - test_task: test-bundled-gems
-          - test_task: check
-            os: ubuntu-24.04
-          - test_task: check
-            os: ubuntu-24.04-arm
+        exclude:
+          - os: ubuntu-24.04-ppc64le
+            upstream: false
+          - os: ubuntu-24.04-s390x
+            upstream: false
       fail-fast: false
 
     env:
@@ -72,7 +89,15 @@ jobs:
         with:
           ruby-version: '3.1'
           bundler: none
-        if: ${{ !endsWith(matrix.os, 'arm') }}
+        if: ${{ !endsWith(matrix.os, 'arm') && !endsWith(matrix.os, 'ppc64le') && !endsWith(matrix.os, 's390x') }}
+
+      # Avoid possible test failures with the zlib applying the following patch
+      # on s390x CPU architecture.
+      # https://github.com/madler/zlib/pull/410
+      - name: Disable DFLTCC
+        run: echo "DFLTCC=0" >> $GITHUB_ENV
+        working-directory:
+        if: ${{ endsWith(matrix.os, 's390x') }}
 
       - uses: ./.github/actions/setup/directories
         with: