Made flash8.Boot.__string_rec public

Currently when targeting flash8, with `-D fdb`, there will be compilation error:
```
C:\HaxeToolkit\haxe\std/haxe/Log.hx:49: characters 14-37 : Cannot access private field __string_rec
C:\HaxeToolkit\haxe\std/haxe/Log.hx:50: characters 98-121 : Cannot access private field __string_rec
```

Since `Boot.__string_rec` is already public for flash9, here I also made the flash8 one public to fix the above issue.