runtime: initial scheduler changes for timers on P's

runtime: initial scheduler changes for timers on P's

Add support to the main scheduler loop for handling timers on P's.
This is not used yet, as timers are not yet put on P's.

Updates #6239
Updates #27707

Change-Id: I6a359df408629f333a9232142ce19e8be8496dae
Reviewed-on: https://go-review.googlesource.com/c/go/+/171826
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>