arch: arm: aarch32: fix z_irq_spurious() implementation

We align the implementation of z_irq_spurious() handler
with the other Zephyr ARCHEs, i.e. we will be calling
directly the ARM-specific fatal error function with
K_ERR_SPURIOUS_IRQ as the error type. This is already
the case for aarch64.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>