runtime: add minimal debug tracing of runtime timers

runtime: add minimal debug tracing of runtime timers

Toggled by a compile-time const, so there should be no
runtime footprint in ordinary builds.

Change-Id: I7751847524f4fda3853388d3e5a18188bd737c27
Reviewed-on: https://go-review.googlesource.com/c/go/+/570336
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Austin Clements <austin@google.com>