Provide an option to not have the default runtime options specified by the wrapper

Summary: Right now we always set these options:

```
    $flags .=
      '-v Eval.EnableHipHopSyntax=true '.
      '-v Eval.EnableHipHopExperimentalSyntax=true '.
      '-v Eval.JitEnableRenameFunction=0 '.
      '-v Eval.GdbSyncChunks=1 '.
      '-v Eval.AllowHhas=true '.
      '';
```

Let's provide an escape hatch just in case we want something just a bit more vanilla

Reviewed By: @jdelong

Differential Revision: D1570066