kernel: syscall_handler: get rid of stdarg
kernel: syscall_handler: get rid of stdarg

We can just implement this as a macro and not needlessly
run afoul of MISRC-C rule 17.1

Fixes: #10012

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

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