Change notation of PCH and PCL registers into PC0 and PC1

This is for forthcoming Atmega2560 support. The Atmega2560 has 24-bit PC
thus, PCH and PCL would become PCH, PCM and PCL (M for Middle). The problem
that in this notation PCM is equivalent to former PCH. This makes compatable
porting kind of difficult, at least for my taste.

Instead PCH becomes PC0 and PCL becomes PC1 (think of the index as order of
bytes pushed to stack when call is performed, PC0 as MSB goes first, PC1
goes to stack second, and for 24-bit PC, PC2 as LDB goes last).