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.
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.
