Code Review Request, 8152237 Support BigInteger.TWO

Hi, Please review the update for the supporting of BigInteger.TWO: http://cr.openjdk.java.net/~xuelei/8152237/webrev/ BigInteger.valueOf(2) is a common BigInteger value used in binary and cryptography operation calculation. The BigInteger.TWO is not exported, and hence BigInteger.valueOf(2) is used instead in applications and JDK components. The export of static BigInteger.TWO can improve performance and simplify existing code. Thanks, Xuelei