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.