trace's reader *g is going to cause write barriers in unfortunate
places, so replace it with a guintptr.

Change-Id: Ie8fb13bb89a78238f9d2a77ec77da703e96df8af
Reviewed-on: https://go-review.googlesource.com/31469
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>