kernel/sched: Use kernel timeouts for timeslice expirations

Rework the fragile and ad-hoc computation of timeslice expirations
into per-CPU struct _timeout objects with regular callbacks.  The
expiration callbacks themselves simply set a per-cpu flag (they might
run on any CPU), which gets checked at the end of the timer ISR on
every CPU.

This simplifies logic and removes a bunch of code.  It also fixes at
least three bugs:

1. As @npitre discovered: On SMP, the number of ticks announced on any
given CPU is going to be a subset of all expired ticks.  This broke
the accounting of timeslice ticks, and effectively meant that
timeslicing only worked on SMP on systems where one CPU could hog all
the announcements, and only on that CPU.

2. The bootstrap path to arm the timer driver after setting the first
timeout in an empty list couldn't take into account
sys_clock_elapsed() ticks, as it didn't know whether it was being
called underneath an existing announce loop.  Now this code is no
longer responsible for knowing anything about time slicing at all.

3. Also on SMP, there was a case where two CPUs timeslicing
simultaneously could stomp on each others' timeouts in
z_set_timeout_expiry(), as neither had a way of knowing what the
other's state was.  CPUs could miss their own expiration and have to
wait for the slice expiration on the other CPU.  Now, timeouts are
global objects with simple expiration times, and there's no need for
that function at all.

Signed-off-by: Andy Ross <andyross@google.com>