runtime: add debug mode to extend _Grunning-without-P windows

runtime: add debug mode to extend _Grunning-without-P windows

This was suggested in CL 646198, and I tested with it, I forgot to push
it so it wasn't merged. I think it might be worth keeping.

Change-Id: Ibf97d969fe7d0eeb365f5f7b1fbeadea3a1076ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/716580
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>