Defer initialization of static fields in java.math.BigInteger

The fix for 6622432: RFE: Performance improvements to java.math.BigDecimal in 6u14 adds two new static fields that are only needed for deserialization. Better to initialize them lazily when needed.

    private static final sun.misc.Unsafe unsafe = sun.misc.Unsafe.getUnsafe();
    private static final long signumOffset;
    private static final long magOffset;
    static {
        try {
            signumOffset = unsafe.objectFieldOffset
                (BigInteger.class.getDeclaredField("signum"));
            magOffset = unsafe.objectFieldOffset
                (BigInteger.class.getDeclaredField("mag"));
        } catch (Exception ex) {
            throw new Error(ex);
        }
    }


zeros is another static field used by the toString() method and it can also be lazily initialized.