arch: arm: common Armv8-M support

This PR includes the required changes in order to support
conditional compilation for Armv8-M architecture. Two
variants of the Armv8-M architecture are defined:
- the Armv8-M Baseline (backwards compatible with ARMv6-M),
- the Armv8-M Mainline (backwards compatible with ARMv7-M).

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