pic32: Fix context switch
pic32: Fix context switch

At some point task_t was modified and keeps stack
bottom instead of stack top.
t_stacktop -> t_stackbottom

For PIC32 FPU context is stored at the very top of stack
(CPU context is stored at current SP).
Context switch code was not updated and still was
using same storage from task_t that was changed.
Lack of update resulted in writes to random places.

This change computes stack top to restore functionality.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core