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

x86: Declare node_info as VISIBLE

node_info is referenced in the linker script and so we must declare it as externally
visible to prevent compiling from eliding it

x64: SKIM window to mitigate Meltdown (CVE-2017-5754) on x86-64

Introduces a kernel option that, when enabled, reduces the kernel window in a user address
space to just be Static Kernel Image and Microstate (SKIM), instead of the full kernel
address space. This isolates the important kernel data from the user preventing a
Meltdown style attack being able to violate secrecy. The kernel text and read only data,
i.e. anything that is static from boot, is not secret and can be allowed in the SKIM window
and potentially read by the user. Additionally to switch to and from the actual kernel
address space a small amount of state needs to also be in the SKIM window.

This is only an implementation for x86-64, although the same design is applicable to ia32

x64: Align and pad node_info

node_info is a per-core data structure and should have always been padded and aligned to
a cache line to ensure there is no false sharing of cache lines

x64: Remove IRQ stack from per core data structure

The IRQ stack represents state that needs to be available in all contexts, both when user
code is running and when kernel code is running. Separating its definition from all the
other mode state provides the option in the future for treating it differently.

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

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