Explicitly create a struct definition for irq_t on SMP Arm
configurations. This makes it a lot harder to mistakenly use the wrong
irq encoding when moving an irq between a cnode index and hardware irq
number / core. A couple areas where this was being handled incorrectly
was fixed as part of the refactor. When performing an ipi for masking
PPI interrupts, the idx encoding is used as it fits into a single word.
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