arm: cortex_m: add vector table padding for null pointer detection

Padding inserted after the (first-stage) vector table,
so that the Zephyr image does not attempt to use the
area which we reserve to detect null pointer dereferencing
(0x0 - <size>). If the end of the vector table section is
higher than the upper end of the reserved area, no padding
 will be added. Note also that the padding will be added
only once, to the first stage vector table, even if the current
snipped is included multiple times (this is for a corner case,
when we want to use this feature together with SW Vector Relaying
on MCUs without VTOR but with an MPU present).

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