MD5_SHIFT_21
GG(a, b, c, d, x_1, MD5_SHIFT_21, MD5_CONST_e(16)); /* 17 */
GG(a, b, c, d, x_5, MD5_SHIFT_21, MD5_CONST_e(20)); /* 21 */
GG(a, b, c, d, x_9, MD5_SHIFT_21, MD5_CONST_e(24)); /* 25 */
GG(a, b, c, d, x_13, MD5_SHIFT_21, MD5_CONST_e(28)); /* 29 */