arm: cortex_m: Use new ARMV6_M_ARMV8_M_BASELINE in vector relay

The old ARMV6_M Kconfig option has been removed, and so to correctly set
the dependencies for SW_VECTOR_RELAY we need to use the new
ARMV6_M_ARMV8_M_BASELINE.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>