signal.c: directly deliver non-handlable signals

* signal.c (rb_f_kill): directly deliver signals which cannot be
  handled, i.e., SIGKILL and SIGSTOP.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45917 b2dd03c8-39d4-4d8f-98ff-823fe69b080e