• thread_pthread.h, thread_win32.h: rename rb_thread_id_t to
  rb_nativethread_id_t.
• thread_pthread.c, vm_core.h: use rb_nativethread_id_t.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42136 b2dd03c8-39d4-4d8f-98ff-823fe69b080e