Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

Remove multiple stack definitions

This commit changes the stack so that it is declared a single time in stack.c,
with the correct attributes and alignment, instead of being declared in stack.h
and relying on the linker to collapse the multiple declarations.

Unify kernel stack definition/declaration and share it between architecures/modes

Rather than defining a kernel stack in random places for each
architecure for each mode (32/64) and for single/SMP modes, make the
stack definition shareable between all of the above. This is also useful
for the future ARM SMP work.