arm: cortex_m: linker.ld: fix relocate by moving _vector_start

The _vector_start was placed before the CONFIG_TEXT_SECTION_OFFSET, thus
adding the offset in the relocated vector table, making the table
invalid when relocated with a non null CONFIG_TEXT_SECTION_OFFSET.

This was tested using MCUboot with a 0x200 offset for the image header.

Fixes: eb48a0a7 ("arm: armv6-m: Support relocating vector table")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>