runtime: don't panic on racy use of timers

runtime: don't panic on racy use of timers

If we see a racy use of timers, as in concurrent calls to Timer.Reset,
do the operations in an unpredictable order, rather than crashing.

Fixes #37400

Change-Id: Idbac295df2dfd551b6d762909d5040fc532c1b34
Reviewed-on: https://go-review.googlesource.com/c/go/+/221077
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>