arm: fatal: log which IRQn is triggering on spurious IRQs

LOG which IRQn line is triggering on spurious IRQs as this makes it
much easier to debug spurious IRQs.

The new logs with this patch looks like:

<err> os: Unhandled IRQn: 227
<err> os: >>> ZEPHYR FATAL ERROR 1: Unhandled interrupt on CPU 0
<err> os: Current thread: 0x20032c20 (unknown)
<err> os: Halting system

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>