signal.c: get rid of deadlock by discarded signals

* signal.c (rb_f_kill): get rid of deadlock as unhandled and
  discarded signals do not make interrupt_cond signaled.
  based on the patch by Kazuki Tsujimoto at [ruby-dev:48606].
  [Bug #9820]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47832 b2dd03c8-39d4-4d8f-98ff-823fe69b080e