host/hardware_irq fails on a debug build

[rhulme]

check_irq_param in irq.h and get_user_irq_claim_index in irq.c have assertions, which refer to PICO_NUM_VTABLE_IRQS. This is not defined for a host build.

In release builds, the assertion defines to nothing, so no build error occurs.

For real hardware, PICO_NUM_VTABLE_IRQS usually uses NUM_IRQS, which in turn in defined in one of the chip-specific platform_defs.h files.