Undefining a data variable should also undefine the symbol at the same address

[jeffli678]

It is good that we can undefine a variable now. However, it does not undefine the symbol at the same address and there is no obvious way to undefine the symbol. Besides, also keep in mind how should we deal with undefine function at the same time.

[plafosse]

This is very similar to #1910