[llvm] [clang] [SystemZ][z/OS] This change adds support for the PPA2 section in zOS (PR #68926)

[via cfe-commits]

dyung wrote:

@ysyeda the test you added, `clang/test/CodeGen/SystemX/systemz-ppa2.c` fails if the SystemZ target (I'm guessing) is not available. Can you add the proper `REQUIRES` line or fix the test to work without that architecture present?

https://lab.llvm.org/buildbot/#/builders/139/builds/54293
```
RUN: at line 13: /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang -cc1 -internal-isystem /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/lib/clang/18/include -nostdsysteminc -triple s390x-ibm-zos -xc -S -o - /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/CodeGen/SystemZ/systemz-ppa2.c | /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/CodeGen/SystemZ/systemz-ppa2.c --check-prefix CHECK-C
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang -cc1 -internal-isystem /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/lib/clang/18/include -nostdsysteminc -triple s390x-ibm-zos -xc -S -o - /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/CodeGen/SystemZ/systemz-ppa2.c
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/FileCheck /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/CodeGen/SystemZ/systemz-ppa2.c --check-prefix CHECK-C
error: unable to create target: 'No available targets are compatible with triple "s390x-ibm-zos"'
```

https://github.com/llvm/llvm-project/pull/68926