CI: Migrate Travis CI ppc64le/s390x cases to the GitHub Actions.

Add a job for the ppc64le/s390x cases using GitHub Actions ppc64le/s390x
service.
https://github.com/IBM/actionspz

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

Author: junaruga

.github/workflows/main.yml

@@ -78,6 +78,27 @@ jobs:
       run: bundle exec rake
       shell: bash
 
+  build-ibm:
+    if: github.repository == 'ruby/prism'
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - ubuntu-24.04-ppc64le
+          - ubuntu-24.04-s390x
+    runs-on: ${{ matrix.os }}
+    steps:
+    - uses: actions/checkout@v4
+    - name: Set up Ruby
+      run: |
+        sudo apt-get update
+        sudo apt-get install ruby-full bundler libyaml-dev
+    - name: Install dependencies
+      run: sudo bundle install --jobs 2
+    - name: Run Ruby tests
+      run: bundle exec rake
+      shell: bash
+
   build-ffi:
     strategy:
       fail-fast: false