[OpenMP] Disable workshare_chunk.c test case on SystemZ

The test added by https://github.com/llvm/llvm-project/pull/83261
has been consistently failing.  Mark as UNSUPPORTED just like on
x86_64 and aarch64.

GitOrigin-RevId: fe1341248d7dd9abd0545ddb48373756645c3f9f
1 file changed