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.

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: Rename global paging structures to kernel

Currently the kernel address space and paging structures are used as the global ones.
This commit renames the paging structures from Global to Kernel to reflect this and
allows for separate global structures to be introduced in the future.

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.

Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

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

x64: Add x86_64 support