mpi: Avoid data-dependent timing variations in mpi_powm.
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mpi: Avoid data-dependent timing variations in mpi_powm.

* include/mpi.h, mpi/mpiutils.c (mpi_set_cond): New.
* mpi/mpi-pow.c (SIZE_PRECOMP): Rename from SIZE_B_2I3.
(mpi_powm): Access all data in the table and use 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.