gccgo: does not recognize unused import

[dvyukov]

gccgo successfully compiles the following program:

package a
import  _ "fmt"
import "fmt"

Both gc and gotype say:

"fmt" imported but not used

They seem to be right.
gofrontend is on commit 81810917af7ba19e1f9f8efc8b1989f7d6419d30

[dvyukov]

@ianlancetaylor @griesemer

[gopherbot]

CL https://golang.org/cl/14259 mentions this issue.