MIPS64: Implement intrinsics from java.lang.String:

- char charAt(int index)
- int compareTo(String anotherString)
- int indexOf(int ch)
- int indexOf(int ch, int fromIndex)
- String(byte[] bytes)
- String(char[] value)
- String(String original)

Change-Id: I9c309b62580b42ae08a09cb1c2b4ebd1a203c5d2