To implement sun.nio.cs.ArrayDecoder/ArrayEncoder in UTF8 charset to speed up String.getBytes() and String.toCharArray(). This is the follow up for cr#6636323, in which all the singlebyte charasets have been optimized to implement sun.nio.cs.ArrayDecoder/Encoder to speed up the String.getBytes()/toCharArray().
See http://blogs.sun.com/xuemingshen/entry/faster_new_string_bytes_cs for details.