runtime: restore nacl timens

Deleted in cl/123700044.
I am not sure whether I need to restore it,
or delete rest of the uses...

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/129580043

src/pkg/runtime/time.go

@@ -34,6 +34,9 @@ var timers struct {
 	t            []*timer
 }
 
+// nacl fake time support.
+var timens int64
+
 // Package time APIs.
 // Godoc uses the comments in package time, not these.