cmd/compile: accepts invalid identifiers

Gc successfully compiles the following program:

``` go
package a
var ۶ = 0
```

It must reject it, as U+06F6 is not classified as Letter (it is a digit):
http://graphemica.com/%DB%B6

go/ast parser correctly rejects the program with:

```
test.go:2:5: illegal character U+06F6 '۶'
```
