arch: arm: Do not enable SW_VECTOR_RELAY with M0+ and VTOR

Whenever a Cortex-M0+ supports the VTOR register it makes no sense to
use the software vector relay mechanism. Therefore change the logic so
that SW_VECTOR_RELAY does not get enabled whenever a VTOR register is
present, but enable it if an M0+ has no VTOR.

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