sync: internal dynamically sized lock-free queue for sync.Pool

This adds a dynamically sized, lock-free, single-producer,
multi-consumer queue that will be used in the new Pool stealing
implementation. It's built on top of the fixed-size queue added in the
previous CL.

For #22950, #22331.

Change-Id: Ifc0ca3895bec7e7f9289ba9fb7dd0332bf96ba5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/166958


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>