[SystemZ] Fix failures after D32542
This commit made ubsan use the fast unwinder.  On SystemZ this requires
test cases to be compiled with -mbackchain.  That was already done for
asan, but not ubsan.  Add the flag for ubsan as well.
@@ -38,6 +38,9 @@ else:
lit_config.fatal("Unknown UBSan test mode: %r" % ubsan_lit_test_mode)
# Platform-specific default for lit tests.
+if config.target_arch == 's390x':
+ # On SystemZ we need -mbackchain to make the fast unwinder work.
+ clang_ubsan_cflags.append("-mbackchain")
if config.host_os == 'Darwin':
# On Darwin, we default to `abort_on_error=1`, which would make tests run
# much slower. Let's override this and run lit tests with 'abort_on_error=0'.
