os/signal: avoid race between Stop and receiving on channel

os/signal: avoid race between Stop and receiving on channel

When Stop is called on a channel, wait until all signals have been
delivered to the channel before returning.

Use atomic operations in sigqueue to communicate more reliably between
the os/signal goroutine and the signal handler.

Fixes #14571

Change-Id: I6c5a9eea1cff85e37a34dffe96f4bb2699e12c6e
Reviewed-on: https://go-review.googlesource.com/46003
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>