runtime: use only per-P gcWork

runtime: use only per-P gcWork

Currently most uses of gcWork use the per-P gcWork, but there are two
places that still use a stack-based gcWork. Simplify things by making
these instead use the per-P gcWork.

Change-Id: I712d012cce9dd5757c8541824e9641ac1c2a329c
Reviewed-on: https://go-review.googlesource.com/19636
Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>