gccgo: treats nil as type

[dvyukov]

gccgo successfully compiles the following program:

package n
func f(a nil)

It must not.
gcc version 6.0.0 2015070 (experimental) (GCC)

[dvyukov]

@paranoiacblack

[gopherbot]

CL https://golang.org/cl/12662 mentions this issue.