thread_pthread: use POSIX timer or thread to get rid of races

This closes race condition where GVL is uncontended and a thread
receives a signal immediately before calling the blocking
function when releasing GVL:

	1) check interrupts
	2) release GVL
	3) blocking function

If signal fires after 1) but before 3), that thread may never
wake up if GVL is uncontended

We also need to wakeup the ubf_list unconditionally on
gvl_yield; because two threads can be yielding to each other
while waiting on IO#close while waiting on threads in IO#read or
IO#gets.

[ruby-core:88360] [Misc #14937]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64353 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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