cmd/asm: unactionable "invalid local variable type 0"

cmd/asm produces the following error message on the program:

```
TEXT T(SB),$0
TYPE
```

```
asm: T: invalid local variable type 0
```

The error message should contain file name and line number, otherwise it is unactionable.

go version devel +5512ac2 Wed Sep 16 17:56:14 2015 +0000 linux/amd64
