runtime: update a few "onM"s in comments to say "systemstack"

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>

src/runtime/mgc.go

@@ -1390,7 +1390,7 @@ func gcinit() {
 	memstats.next_gc = heapminimum
 }
 
-// Called from malloc.go using onM, stopping and starting the world handled in caller.
+// Called from malloc.go using systemstack, stopping and starting the world handled in caller.
 //go:nowritebarrier
 func gc_m(start_time int64, eagersweep bool) {
 	_g_ := getg()
@@ -1419,7 +1419,7 @@ func clearCheckmarks() {
 	}
 }
 
-// Called from malloc.go using onM.
+// Called from malloc.go using systemstack.
 // The world is stopped. Rerun the scan and mark phases
 // using the bitMarkedCheck bit instead of the
 // bitMarked bit. If the marking encounters an