arch: arm: cortex-m: fix the accounted size of IRQ stack in reset.S

This patch is simply adding the guard area (if applicable) to
the calculations for the size of the interrupt stack in reset.S
for ARM Cortex-M architecture. If exists, the GUARD area is
always reserved aside from CONFIG_ISR_STACK_SIZE, since the
interrupt stack is defined using the K_KERNEL_STACK_DEFINE.

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