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>