Mega2560 3 byte PC and gcc 2 byte PC

Hi.

This is my new proposal about gcc giving us a 2 byte PC.
It propose to initialize the PC part of stack, only with 2 byte to the function pointer and to put the task functions in the lower 128Kb part of the memory as we discuses in 
https://github.com/DuinOS/DuinOS/pull/8