Make `RegExp.prototype.flags` more spec-compliant

Note that this still isn’t fully spec-compliant, since it shouldn’t be configurable.
Per https://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-ecmascript-standard-built-in-objects and https://twitter.com/awbjs/status/543586880835428352 this should indeed be configurable.