py: Get optional VM stack overflow check compiling and working again.

Changes to the layout of the bytecode header meant that this debug code was
no longer compiling.  This is now fixed and a new compile-time option is
introduced, MICROPY_DEBUG_VM_STACK_OVERFLOW, to turn on this feature (which
is disabled by default).  This option is needed because more than one file
needs to cooperate to make this check work.