Optimize key setup for GCM mode.

GCM mode requires GHASH for 2 different operations: one for
the data (AD + ciphertext) and one for the IV.

Construction of tables to speed-up GHASH is very expensive
and it is worth doing only for the data, not for the IV.

This patch ensures that the GHASH for the IV does not
use tables, with a ~40% faster key setup.