replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

Fix trivial comment typos.

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.