kernel: Restore CONFIG_MULTITHREADING=n behavior
kernel: Restore CONFIG_MULTITHREADING=n behavior

The prepare_multithreading()/switch_to_main_thread() steps were being
done unconditionally, when with multhreading disabled we want to jump
straight into the main thread on the existing stack.

Needless to say, that doesn't work well.  Fixes #8361.

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

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