Override complete compiler tool paths for chipKIT.

Previously we were setting the AVR tools path to the PIC32 tools path because it
made grabbing the compiling tools easier. Consequently, it made finding the
avrdude path much harder, especially since the avrdude files are in different
locations in the Linux distributions of MPIDE and Arduino.

Instead, we set the AVR tools path to the *correct* path (where *AVR* dude
lives), and totally override the CC, CXX, etc. paths to point to their PIC32
equivalents.

Eventually this will probably get merged with whatever changes made to handle the multiple platforms supported by Arduino 1.5 (AVR and ARM). The flashing tool and compiling tools are conflated in the current release, but we may need to split it up going forward based on this example (i.e. the chipKIT uses pic32 tools but still uses avrdude to flash).