ec: Use mpi_mulm instead of mpi_powm.
* mpi/ec.c (ec_pow2): New. (ec_powm): Remove call to mpi_abs. (dup_point_weierstrass, dup_point_twistededwards) (add_points_weierstrass, add_points_twistededwards) (_gcry_mpi_ec_curve_point): Use ec_pow2.
- Signed-off-by: Werner Koch <wk@gnupg.org>