[release-branch.go1.14] runtime: don't call wakeNetPoller during timerModifying

Reduce the length of time that other timer functions can see timerModifying.
In particular avoid system calls.

For #38023
Fixes #38051

Change-Id: I1b61229c668e6085d9ee6dca9488a90055386c36
Reviewed-on: https://go-review.googlesource.com/c/go/+/224902


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
(cherry picked from commit 355f53f0)
Reviewed-on: https://go-review.googlesource.com/c/go/+/225277