arch: arm: enforce double-word stack alignment on exception entry

This commit enforces default double-word stack alignmnet
on exception entry for Cortex-M3 and Cortex-M4 MCUs. The
patch ensures that we have consistent behavior in all
Cortex-M MCUs (double-world stack alignment on exception
entry is enforced by default in ARMv6, ARMv8, and Cortex-M7
processors).

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