RCU pointer fixes.

- Use __typeof__, not typeof.
- Use membar_exit, not membar_producer.
  => We want a store-release, not store/store barrier.
  => Consider:
      foo->x = 42;
      assert(foo->x == 42);
      rcu_assign_pointer(globalfoo, foo);
    The foo->x load can be reordered past the store/store barrier
    (membar_producer), and happen after another thread has clobbered
    foo->x.
- Put the evaluation of the right-hand side before the barrier.

(riastradh)