arch: arm: re-implement z_arch_is_in_isr

We re-implement the z_arch_is_in_isr function
so it aligns with the implementation for other
ARCHEs, i.e. returning false whenever any IRQ
or system exception is active.

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