thread_pthread: prefer rb_nativethread* types/functions

This will make it easier for us to try alternative mutex/condvar
implementations while still using pthreads for thread management.
[Feature #10134]

* thread_pthread.h: define RB_NATIVETHREAD_LOCK_INIT and
  RB_NATIVETHREAD_COND_INIT macros

* thread_pthread.c (native_mutex_lock, native_mutex_unlock,
  native_mutex_trylock, native_mutex_initialize,
  native_mutex_destroy, native_cond_wait):
  use rb_nativethread_lock_t instead of pthread_mutex_t

* thread_pthread.c (native_mutex_debug): make argument type-agnostic
  to avoid later cast.

* thread_pthread.c (register_cached_thread_and_wait):
  replace PTHREAD_COND_INITIALIZER with RB_NATIVETHREAD_COND_INIT,
  use native_mutex_{lock,unlock}

* thread_pthread.c (use_cached_thread):
  use native_mutex_{lock,unlock}

* thread_pthread.c (native_sleep):
  use rb_nativethread_lock_t to match th->interrupt_lock,
  use native_mutex_{lock,unlock}

* thread_pthread.c (timer_thread_lock): use rb_nativethread_lock_t type

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47185 b2dd03c8-39d4-4d8f-98ff-823fe69b080e