mpi: Avoid data-dependent timing variations in mpi_powm.

* mpi/mpi-pow.c (mpi_powm): Access all data in the table by
mpi_set_cond.

Access to the precomputed table was indexed by a portion of EXPO,
which could be mounted by a side channel attack. This change fixes
this particular data-dependent access pattern.

Cherry-picked from commit 5e72b6c76ebee720f69b8a5c212f52d38eb50287
in LIBGCRYPT-1-6-BRANCH.