Commit 656be317 replaced onM with systemstack, but missed updating a
few comments that still referred to onM.  Update these.

Change-Id: I0efb017e9a66ea0adebb6e1da6e518ee11263f69
Reviewed-on: https://go-review.googlesource.com/3664Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>