build: fix make.rc on Plan 9
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://codereview.appspot.com/9611045

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang