We need to ensure Signal.trap handlers can function if the main
thread is sleeping after a subthread has grabbed sigwait_fd,
but later exited.

Consider the following timeline:

main_thread             sub-thread
-----------------------------------------
Signal.trap() { ... }
                        get sigwait_fd
                        ppoll on sigwait_fd
native_cond_sleep
(via pthread_cond_wait)
                        ppoll times-out
			put sigwait_fd
                        sub-thread exits

only thread alive
SIGNAL HITS

The problem is pthread_cond_wait cannot return EINTR,
so we can never run the Signal.trap handler.  So we
will avoid using native_cond_sleep in the main thread
and always use ppoll to sleep when in the main thread.
This can guarantee the main thread remains aware of
signals; even if it cannot safely read off sigwait_fd

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64538 b2dd03c8-39d4-4d8f-98ff-823fe69b080e