cmd/compile: provide line number for cgo directive error (fix a TODO)

Also: Remove double "go:" prefix in related error message.

Fixes #18882.

Change-Id: Ifbbd8e2f7529b43f035d3dbf7ca4a91f212bc6b6
Reviewed-on: https://go-review.googlesource.com/36121


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>