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