cgo: better error for no C symbols
cgo: better error for no C symbols

R=rsc, iant
CC=golang-dev, graeme.perrow
http://codereview.appspot.com/1193046

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

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