test: match gccgo error messages for blank1.go

blank1.go:10:9: error: invalid package name _
blank1.go:17:2: error: cannot use _ as value
blank1.go:18:7: error: cannot use _ as value
blank1.go:20:8: error: invalid use of ?_?

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/14088044