mpi:ec: Don't use mpi_powm for small exponent.
* mpi/ec.c (ec_pow3): Remove. (_gcry_mpi_ec_curve_point): Don't use ec_pow3. (dup_point_weierstrass): Use ec_pow2 twice for Z^4. (add_points_weierstrass): Don't use ec_powm.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>