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.

arm: remote IPI call support for VIRQS

Added support for injecting remote IPI calls towards given VCPU's
on SMP configured systems. This introudcing a new type of
IpiRemoteCall and handlers for updating the vgic state on
incoming/outgoing IPI's.

Co-authored-by: Yanyan Shen <yanyan.shen@data61.csiro.au>

style: consistently attach return type

Add attach-return-type to astyle

SMP: Introduce ENABLE_SMP_SUPPORT

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

SMP: allow an architecture to provide its own optimised IPI send function

SELFOUR-747: SMP/x86: generalize arch-independent IPI code

Fix licenses (the rest)

smp: fix bugs when stalling remote core

- Restart TCB from inside the lock if it is waiting for anything other than IRQ
- Only replace the TCB with idle thread if it is in ThreadState_RunningVM state

Also, this makes the design generic to be shared with arm.

s/D61/DATA61/ in license headers for consistency

SMP: Abstract architecture/platform independent parts of smp.h and ipi.h

This commit is a re-arrangement of SMP directory structure to make it
easier for other architectures/platforms (in general) and ARM (in
particular) to add SMP support.

* new include/smp directory to act as a centralised container of
"shared" architecture-independent SMP headers.
This makes it clearer what's needed for other architecture/platform to
support SMP.
* Each platform can define its own unique ipi.[h|c] that make sense,
since ipi implementation is SoC/platform dependent.