Currently the g.waiting list created by a select is in poll order.
However, nothing depends on this, and we're going to need access to
the channel lock order in other places shortly, so modify select to
put the waiting list in channel lock order.

For #12967.

Change-Id: If0d38816216ecbb37a36624d9b25dd96e0a775ec
Reviewed-on: https://go-review.googlesource.com/20037Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Austin Clements <austin@google.com>