kernel/sched: Adjust locking in z_swap()

Swap was originally written to use the scheduler lock just to select a
new thread, but it would be nice to be able to rely on scheduler
atomicity later in the process (in particular it would be nice if the
assignment to cpu.current could be seen atomically).  Rework the code
a bit so that swap takes the lock itself and holds it until just
before the call to arch_switch().

Note that the local interrupt mask has always been required to be held
across the swap, so extending the lock here has no effect on latency
at all on uniprocessor setups, and even on SMP only affects average
latency and not worst case.

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