Use a real Ruby mutex in rb_io_close_wait_list (#7884)

Because a thread calling IO#close now blocks in a native condvar wait,
it's possible for there to be _no_ threads left to actually handle
incoming signals/ubf calls/etc.

This manifested as failing tests on Solaris 10 (SPARC), because:

* One thread called IO#close, which sent a SIGVTALRM to the other
  thread to interrupt it, and then waited on the condvar to be notified
  that the reading thread was done.
* One thread was calling IO#read, but it hadn't yet reached the actual
  call to select(2) when the SIGVTALRM arrived, so it never unblocked
  itself.

This results in a deadlock.

The fix is to use a real Ruby mutex for the close lock; that way, the
closing thread goes into sigwait-sleep and can keep trying to interrupt
the select(2) thread.

See the discussion in: https://github.com/ruby/ruby/pull/7865/

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