kernel/thread: Don't clobber arch initialization of switch_handle
kernel/thread: Don't clobber arch initialization of switch_handle

The recent synchronization work required that the kernel guarantee
switch_handle is non-null, but it did it in a way that works for ARC
and x86_64 but would clobber the work xtensa had already done to
populate that field.

There's no point: just make this an assert, as it's always been the
arch layer's job.

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

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