hurd: Avoid PLTs for __pthread_get/setspecific

* sysdeps/htl/pthreadP.h [IS_IN (libpthread)] (__pthread_getspecific,
__pthread_setspecific): Add hidden proto.
* sysdeps/htl/pt-getspecific.c (__pthread_getspecific): Add hidden def.
* sysdeps/htl/pt-setspecific.c (__pthread_setspecific): Add hidden def.