cmd/compile: symbol ""._.args_stackmap listed multiple times

gc fails on the following program:

``` go
package a
func _()
func _()
```

```
duplicate 00000 (/tmp/111.go:3) GLOBL   ""._.args_stackmap(SB), $8
2015/07/13 16:28:26 symbol ""._.args_stackmap listed multiple times
```

I think that formally it is a legal program that should be compiled without errors (gotype agrees).

go version devel +9b04852 Sat Jul 11 00:08:50 2015 +0000 linux/amd64
