mpi: Add _gcry_mpih_mul_sec and use if for gcry_mpi_mul.
* mpi/mpi-internal.h (_gcry_mpih_mul_sec): New. * mpi/mpi-mul.c (_gcry_mpi_mul): Use _gcry_mpih_mul_sec. * mpi/mpih-mul.c (_gcry_mpih_mul_sec): New.
- GnuPG-bug-id: T7490
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>