PhantomReference should not permit referent access

The documentation for PhantomReference says

  "... the referent of a phantom reference may not be retrieved: The
  {@code get} method of a phantom reference always returns {@code null}."

and the documentation for get() says

  "Returns this reference object's referent. Because the referent of
  a phantom reference is always inaccessible, this method always
  returns {@code null}."

In other words, those do not merely say that PhantomReference.get() returns null. Rather, they say there is no access path to the referent and, as a consequence, get() returns null.

However, the current implementation allows access to the referent of a PhantomReference by either reflection (ultimately, Unsafe_GetObject or Unsafe_GetObjectVolatile) or JNI (jni_GetObjectField). Neither of these access paths does anything to recognize the field being accessed is a PhantomReference referent and block the access.

Note that these accessors already need to recognize referent accesses in order to provide the read barrier needed by G1.