disable MN threads on s390s-linux

I check the s390s-linux/Ubuntu system and I found that MN threads
doesn't work with:

* function inlining (-O0 doesn't repro)
* Thread local specifier (`_Thread_local`.
  With `pthread_get_specific works)
  (it is not an issue of `__tls_get_addr()` written in thread.c)
* swap context with ucontext (coroutine/ucontext)

I couldn't find out what is the root cause of this issue but
disable MN threads to make CI healthy.

configure.ac

@@ -4364,6 +4364,10 @@ AS_IF([test "${universal_binary-no}" = yes ], [
         arch="${target_cpu}-${target_os}"
     ])
     AC_DEFINE_UNQUOTED(RUBY_PLATFORM, "$arch")
+
+    AS_IF([test "$arch" = "s390x-linux"], [
+       AC_DEFINE_UNQUOTED(USE_MN_THREADS, 0)
+    ])
 ])
 
 unset sitearch