gccgo: allows multiple declarations of a function

[dvyukov]

gcc successfully compiles the following program:

package a
func n()
func n()

Both gc and go/types reject it. Compilers need to agree.

gcc version 6.0.0 2015070 (experimental) (GCC)

[dvyukov]

@paranoiacblack

[gopherbot]

CL https://golang.org/cl/12508 mentions this issue.