kernel/timeout: Serialize handler callbacks on SMP

On multiprocessor systems, it's routine to enter sys_clock_announce()
in parallel (the driver will generally announce zero ticks on all but
one cpu).

When that happens, each call will independently enter the loop over
the timeout list.  The access is correctly synchronized, so the list
handling is correct.  But the lock is RELEASED around the invocation
of the callback, which means that the individual callbacks may
interleave between cpus.  That means that individual
application-provided callbacks may be executed in parallel, which to
the app is indistinguishable from "out of order".

That's surprising and error-prone.  Don't do it.  Place a secondary
outer spinlock around the announce loop (but not the timeslicing
handling) to correctly serialize the timeout handling on a single cpu.

(It should be noted that this was discovered not because of a timeout
callback race, but because the resulting simultaneous calls to
sys_clock_set_timeout from separate cores seems to cause extremely
high latency excursions on intel_adsp hardware using the cavs_timer
driver.  That hardware issue is still poorly understood, but this fix
is desirable regardless.)

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