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.

Explicit casts between enum types in IPI

Depending on the platform, IPI has a slightly different interface.
Explicitly cast from the more speific enum type to the generic
and vice versa when calling in and out of generic code.

Another possibility would be to typedef a specific enum on RISCV
platforms that aliases the one currently used. Then casting would only
have to take place when passing args to function calls.

style: consistently align pointer with name

Run astyle with align-pointer=name

SMP: Introduce ENABLE_SMP_SUPPORT

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

x86/SMP: Convert IRQ number to vector number when sending an IPI

ipi_send_mask is defined as:

void ipi_send_mask(irq_t ipi, word_t mask, bool_t isBlocking);

It expects an IRQ IPI number, not an interrupt number in the case of x86.

This commit:

1- Removes the confusion between IRQ and interrupt numbers by explicitly
converting IRQ to interrupt numbers in x86 implementation.

2- Makes the smp/ipi.c more generic to be shared with ARM, which only has
IRQ numbers for IPIs (send/receive).

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/x86: convention - put all ipi.* files under smp/ directory

This follows the convention of other higher-level ipi.* smp.* files
put under smp/. Also upcoming ARM files.