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 <andrew.j.ross@intel.com>