build: fix make.rc on Plan 9

Set $status as null to prevent rc from exiting
on the last --no-banner argument checking when
used with rc -e. It allows all.rc to not exit
before executing run.rc

R=golang-dev, lucio.dere, rsc
CC=golang-dev
https://golang.org/cl/9611045

src/make.rc

@@ -95,3 +95,5 @@ rm -f $GOTOOLDIR/go_bootstrap
 
 if(! ~ $1 --no-banner)
 	$GOTOOLDIR/dist banner
+
+status=''