arch: arm: core: cortex_m: Make relocate_vector_table weak

Allow relocate_vector_table() to be overwritten. This is necessary if
the IRQ vector table is handled proprietary, e.g. before the zephyr
kernel is initialized. In this case we want this function to be empty to
avoid overriding the previous configuration.

Co-authored-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
Signed-off-by: Eivind Jølsgard <eivind.jolsgard@nordicsemi.no>