runtime: record stub netpoll initialization, add lock around note
runtime: record stub netpoll initialization, add lock around note

This fixes the Plan 9 support for the new timer code.

Updates #6239
Updates #27707

Change-Id: Ia498c399b8924910b97fcde07545fae3588aad47
Reviewed-on: https://go-review.googlesource.com/c/go/+/204045
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

cloudflare/go,golang/go,nawawi/go,akutz/go,akutz/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,golang/go,akutz/go,akutz/go,cloudflare/go,golang/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go