arm: fix the VTOR alignment requirement for Baseline Cortex-M

Baseline Cortex-M requires VTOR to be aligned on 64-word
boundary. That is because bit-7 of VTOR is also RAZ/WI.
The commit updates the vector table section alignment for
Baseline Cortex-M to reflect the implementation constraint.

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