arch: arm: cortex-m: hw stack protection under no multi-threading

This commit adds the support for HW Stack Protection when
building Zephyr without support for multi-threading. The
single MPU guard (if the feature is enabled) is set to
guard the Main stack area. The stack fail check is also
updated.

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