tests: benchmarks: sys_kernel: Migrate to K_THREAD_STACK_DECLARE
tests: benchmarks: sys_kernel: Migrate to K_THREAD_STACK_DECLARE

This commit updates all deprecated `K_THREAD_STACK_EXTERN` macro usages
to use the `K_THREAD_STACK_DECLARE` macro instead.

Signed-off-by: Stephanos Ioannidis <dc76e9f0c0006e8f919e0c515c66dbba3982f785@stephanos.io>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr