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

The job runs conditionally only in the original (non-fork) repository.
Because we cannot run the job in the forked repositories. The ppc64le/s390x
runners only work on the registered repositories.

Author: junaruga

.github/workflows/main.yml

@@ -78,6 +78,26 @@ 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
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: head
+        bundler-cache: true
+    - name: Run Ruby tests
+      run: bundle exec rake
+      shell: bash
+
   build-ffi:
     strategy:
       fail-fast: false