timeout.rb: same object across fiber

* lib/timeout.rb (Timeout::ExitException.catch): do not freeze the
  exception for tag, so that the same object can be passed to the
  target fiber without duplication to attach backtrace at raise.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48467 b2dd03c8-39d4-4d8f-98ff-823fe69b080e