arm: cortex_m: fix vector table relocation in non-XIP builds

When VTOR is implemented on the Cortex-M SoC, we can
basically use any address (properly aligned) for the
vector table starting address. We fix the setting of
VTOR in prep_c.c for non-XIP images, in this commit,
so we do not need to always have the vector table be
present at the start of RAM (CONFIG_SRAM_BASE_ADDRESS)
and allow for extra linker sections being placed before
the vector table section.

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