Ruby

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.