kernel/timeout: Cleanup/speedup parallel announce logic

Commit b1182bf8 ("kernel/timeout: Serialize handler callbacks on
SMP") introduced an important fix to timeout handling on
multiprocessor systems, but it did it in a clumsy way by holding a
spinlock across the entire timeout process on all cores (everything
would have to spin until one core finished the list).  The lock also
delays any nested interrupts that might otherwise be delivered, which
breaks our nested_irq_offload case on xtensa+SMP (where contra x86,
the "synchronous" interrupt is sensitive to mask state).

Doing this right turns out not to be so hard: take the timeout lock,
check to see if someone is already iterating
(i.e. "announce_remaining" is non-zero), and if so just increment the
ticks to announce and exit.  The original cpu will then complete the
full timeout list without blocking any others longer than needed to
check the timeout state.

Fixes #44758

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
(cherry picked from commit 0b2ed381)