ia64: Don't clear register_stack_start

r59829 stopped clearing stack_start and enabled the code for
!FIBER_USE_NATIVE, but we need to do the same for register_stack_start
on ia64, otherwise we end up with NULL in cont_save_machine_stack.

Closes: https://github.com/ruby/ruby/pull/2155

cont.c

@@ -621,7 +621,6 @@ cont_save_thread(rb_context_t *cont, rb_thread_t *th)
     sec->machine.stack_end = NULL;
 
 #ifdef __ia64
-    sec->machine.register_stack_start = NULL;
     sec->machine.register_stack_end = NULL;
 #endif
 }