[clang] [llvm] [SystemZ] Add backchain target-feature (PR #71668)

[Ilya Leoshkevich via cfe-commits]

iii-i wrote:

This is now causing a CI failure: https://lab.llvm.org/buildbot/#/builders/139/builds/53143
I think I need something like `// REQUIRES: s390x-registered-target` in the new test. I will open a PR with the fix soon.

https://github.com/llvm/llvm-project/pull/71668