arm: armv6-m: Support relocating vector table

An abnormal crash was encountered in ARMv6-M SoCs that don't have flash
starting at 0.  With Zephyr OS the reason for this crash is that, on
ARMv6-M the system requires an exception vector table at the 0 address.

We implement the relocate_vector_table function to move the vector table
code to address 0 on systems which don't have the start of code already
at 0.

[kumar.gala: reworderd commit message, tweaked how we check if we need
 to copy vector table]

Signed-off-by: Xiaorui Hu <xiaorui.hu@linaro.org>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>