CI: ubuntu.yml: Add GitHub Actions ppc64le case

Note that the default configure option `./configure cppflags=-DRUBY_DEBUG`
causes the following Ractor test and other tests failing. So, we don't set the
option in ppc64le cases.

```
$ make btest BTESTS=bootstraptest/test_ractor.rb
```

You can check the following ticket for details.
https://bugs.ruby-lang.org/issues/21534

.github/workflows/ubuntu.yml

@@ -25,15 +25,13 @@ jobs:
     strategy:
       matrix:
         test_task: [check]
-        configure: ['']
+        configure: ['cppflags=-DRUBY_DEBUG']
         arch: ['']
         os:
+          - ubuntu-22.04
           - 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-ppc64le
           - ubuntu-24.04-s390x
         # The ppc64le/s390x runners work only in the registered repositories.
         # They don't work in forked repositories.
@@ -49,6 +47,12 @@ jobs:
             configure: '--disable-yjit'
           - test_task: check
             configure: '--enable-shared --enable-load-relative'
+          - os: ubuntu-24.04-ppc64le
+            # FIXME Override the configure options with empty, due to some
+            # Ractor tests failing with the "./configure cppflags=-DRUBY_DEBUG"
+            # on ppc64le.
+            # https://bugs.ruby-lang.org/issues/21534
+            configure: ''
           - test_task: test-bundler-parallel
             timeout: 50
           - test_task: test-bundled-gems
@@ -122,9 +126,8 @@ jobs:
       - name: Run configure
         env:
           arch: ${{ matrix.arch }}
-          configure: ${{ matrix.configure }}
         run: >-
-          $SETARCH ../src/configure -C --disable-install-doc ${configure:-cppflags=-DRUBY_DEBUG}
+          $SETARCH ../src/configure -C --disable-install-doc ${{ matrix.configure }}
           ${arch:+--target=$arch-$OSTYPE --host=$arch-$OSTYPE}
 
       - run: $SETARCH make prepare-gems