kernel: arch: Clarify output switch handle requirements in arch_switch

The original intent was that the output handle be written through the
pointer in the second argument, though not all architectures used that
scheme.  As it turns out, that write is becoming a synchronization
signal, so it's no longer optional.

Clarify the documentation in arch_switch() about this requirement, and
add an instruction to the x86_64 context switch to implement it as
original envisioned.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>