arm: cortex_m: linker.ld: fix relocate by moving _vector_start
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: eb48a0a73c11 ("arm: armv6-m: Support relocating vector table")
Signed-off-by: Neil Armstrong <619b324efbf5f7848de055bdc8bef4c0dd7c8ef5@baylibre.com>

Vudentz/zephyr,finikorg/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,mbolivar/zephyr,explora26/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,nashif/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,kraj/zephyr,nashif/zephyr,mbolivar/zephyr,finikorg/zephyr,nashif/zephyr,mbolivar/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,punitvara/zephyr,explora26/zephyr,kraj/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,mbolivar/zephyr,ldts/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,ldts/zephyr,nashif/zephyr,kraj/zephyr,punitvara/zephyr,mbolivar/zephyr,ldts/zephyr,explora26/zephyr,ldts/zephyr,kraj/zephyr,galak/zephyr,zephyriot/zephyr,zephyriot/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr