runtime: change p.runq from []*g to []guintptr

This eliminates many write barriers in the scheduler code that are
unnecessary and will interfere with upcoming changes where the garbage
collector will have to invoke run queue functions in contexts that
must not have write barriers.

Change-Id: I702d0ac99cfd00ffff406e7362917db6a43e7e55
Reviewed-on: https://go-review.googlesource.com/16556


Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Austin Clements <austin@google.com>