Update the RegExp constructor to ECMA-262 v6 (#3538)

The following methods have been implemented:
- RegExpAlloc, based on ECMA-262 v6, 21.2.3.2.1
- RegExpInitialize, based on ECMA-262 v6, 22.2.3.2.2
- RegExpCreate, based on ECMA-262 v6, 21.2.3.2.3

Co-authored-by: Robert Fancsik frobert@inf.u-szeged.hu
JerryScript-DCO-1.0-Signed-off-by: Adam Szilagyi aszilagy@inf.u-szeged.hu

tests/jerry/es2015/regexp-construct.js

@@ -14,12 +14,18 @@
 
 var r = RegExp ("a","gim");
 var r2 = RegExp (r,"gim");
+var r3 = RegExp (r);
 
 assert(r2.source === 'a');
 assert(r2.global === true);
 assert(r2.ignoreCase === true);
 assert(r2.multiline === true);
 
+assert(r3.source === 'a');
+assert(r3.global === true);
+assert(r3.ignoreCase === true);
+assert(r3.multiline === true);
+
 var obj = { get source() { throw 5 }, [Symbol.match] : true }
 
 try {