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.

style: consistently attach return type

Add attach-return-type to astyle

style: consistently align pointer with name

Run astyle with align-pointer=name

style: correct parenthesis padding

Use astyle's unpad-paren to unpad all parentheses that are not included
by pad-header, pad-oper, and pad-comma.

SMP: Introduce ENABLE_SMP_SUPPORT

- Make it more readable and less confusing compared to the 'CONFIG_MAX_NUM_NODES > 1' check

Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

x86: Explicitly define kernel stack size

This commit changes the previous hard coded 4K kernel stack size
to being a configurable power of 2 sized stack

x86: Improve performance of getCurrentCPUIndex

The use of 'volatile' on the asm blocks used by getCurrentCPUIndex
resulted in an inability for the compiler to elide repeated invocations
of getCurrentCPUIndex when inlining. The volatile is not needed
as we already want to claim that these functions are CONST and so
the asm block can be move/reordered/deleted as the compiler wishes

x64: Support multiple kernel stacks

Adds support for per-core kernel stacks through the use of
thread local storage and swapgs. In addition to the main
kernel stack the IRQ stack also needs to be made per core

x86: Split CPU ID management into mode and general

This moves getCurrentCPUIndex into a mode specific smp header, and
moves wrappers that use it into the generap smp header