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.