thread.c: fix deadlock

* thread.c (ruby_kill): get rid of deadlock on signal 0.
  [ruby-dev:47182] [Bug #8137]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39862 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* thread_win32.c (native_sleep): suppress warning.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39863 b2dd03c8-39d4-4d8f-98ff-823fe69b080e