Chipkit 2013 updates

These are the updates required in the Makefile to support the latest version of the chipKIT IDE (MPIDE, available from http://chipkit.net/started/) - version 0023-linux-20130817-test and the accompanying Windows and OS X builds (they have slightly different dates).

There were 2 changes required:

* Add another linker script
* Add a vector table written in assembly to the core lib