mpi: Revise mpi_powm.

* mpi/mpi-pow.c (_gcry_mpi_powm): Rename the table to PRECOMP.

The name of precomputed table was b_2i3 which stands for BASE^(2*I+3).
But it's too cryptic, so, it's renamed. Besides, we needed to
distinguish the case of I==0, that was not good. Since it's OK to
increase the size of table by one, it's BASE^(2*I+1), now.