Cortex M0+ is ARMv6-M. ARMv8 has also an M variant, Cortex M33 is ARMv8-M mainline. ARMv6-M and ARMv8-M have quite a lot of things in common, as v8 is a superset of v6. Or did you mean ARMv8-A? Well, ARMv8-A and ARMv8-M are not compatible intentionally, as the the A variant is strictly 64-bit (some implementations support AArch32 though) and the support for thumb instruction set has been dropped.