• thread_native.h: add rb_nativethread_self() which returns
  current running native thread identifier.
• thread_[pthread|win32].c: implement rb_nativethread_self().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42137 b2dd03c8-39d4-4d8f-98ff-823fe69b080e