This is currently for testing GVL performance in the uncontended
case: IO#write and IO#read unconditionally release GVL for
blocking I/O with pipe.
It will also be interesting to see how this changes if we switch
to M:N threading model.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58556 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
