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

  Move native thread related lines from vm_core.h.
  And declare several functions "rb_nativethread_lock_*",
  manipulate locking.
* common.mk: add thread_native.h.
* thread.c: add functions "rb_nativethread_lock_*".
* thraed.c, thread_[pthread,win32].[ch]: rename rb_thread_lock_t
  to rb_nativethread_lock_t to make it clear that this lock is for
  native thraeds, not for ruby threads.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42133 b2dd03c8-39d4-4d8f-98ff-823fe69b080e