fixup! Add openmp support to System z: Build openpm tests with -mbackchain

Author: iii-i

openmp/runtime/test/lit.cfg

@@ -51,6 +51,8 @@ flags = " -I " + config.test_source_root + \
     " " + config.test_extra_flags
 if config.has_omit_frame_pointer_flag:
     flags += " -fno-omit-frame-pointer"
+if config.target_arch == "s390x":
+    flags += " -mbackchain"
 
 config.test_flags = " -I " + config.omp_header_directory + flags
 config.test_flags_use_compiler_omp_h = flags