ld: fail linking if the top-level package is not main.
ld: fail linking if the top-level package is not main.

This makes {5,6,8}l conform to the spec more tightly.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/4826063

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

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