gccgo: accepts program with invalid identifier (2)

[dvyukov]

gccgo successfully compiles the following program:

package a
var _ *i

while gc and go/types reject it with:

go.go:2: undefined: i
go.go:2:7: undeclared name: i

gccgo must reject it too.
#11522

[dvyukov]

@paranoiacblack

[gopherbot]

CL https://golang.org/cl/13031 mentions this issue.