arm: arch.h: move extern "C" after includes
arm: arch.h: move extern "C" after includes

Shouldn't declare this and then pull in headers, fixes
errors like "util.h:41:1: error: template with C linkage"

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

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