Update ES6 Regexp accessors descriptors (#3726)

In ES6 accessors of built in object are by default configurable.
Also lastIndex property of RegExp object is created when only used.

JerryScript-DCO-1.0-Signed-off-by: Rafal Walczyna r.walczyna@samsung.com

tests/jerry/es2015/regexp-lastindex.js

@@ -18,3 +18,5 @@ result = t.exec("abc   abc");
 assert(result[0] === "abc");
 assert(result.index === 0);
 assert(t.lastIndex === 3);
+
+assert(RegExp.prototype.lastIndex === undefined)