Fix the types of builtin prototype objects (#3663)

In ES2015 many builtin prototypes are no longer valid instances of their
respective classes. This change updates affected prototypes to be
regular objects as required.

JerryScript-DCO-1.0-Signed-off-by: Dániel Bátyai dbatyai@inf.u-szeged.hu

tests/jerry/es2015/regexp-flags.js

@@ -60,3 +60,13 @@ assert (r.lastIndex === 0);
 var r = /abc/yg;
 m = r.exec ("strabcstr");
 assert (m === null);
+
+try {
+  RegExp.prototype.flags;
+  assert (false);
+} catch (e) {
+  assert (e instanceof TypeError);
+}
+
+var flagsProp = Object.getOwnPropertyDescriptor (RegExp.prototype, "flags");
+assert(flagsProp.get.call({}) === '');