* mpi/mpi-pow.c (USE_ALGORITHM_SIMPLE_EXPONENTIATION): New.
(mul_mod) [!USE_ALGORITHM_SIMPLE_EXPONENTIATION]: New.
(mpi_powm) [!USE_ALGORITHM_SIMPLE_EXPONENTIATION]: New implementation
of left-to-right k-ary exponentiation.

Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>

For the Yarom/Falkner flush+reload cache side-channel attack, we
changed the code so that it always calls the multiplication
routine (even if we can skip it to get result).  This results some
performance regression.

This change is for recovering performance with efficient
algorithm.