mpi: Add _gcry_mpih_cmp_lli, Least Leak Intended.

* mpi/mpi-internal.h (_gcry_mpih_cmp_lli): New.
* mpi/mpih-const-time.c (_gcry_mpih_cmp_lli): New.

• Co-authored-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>