runtime: define NSIG to fix plan 9 build
runtime: define NSIG to fix plan 9 build

R=rsc
CC=golang-dev
http://codereview.appspot.com/5677053

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go