Make GHASH more robust against timing attacks.

In order to speed up as much as possible the GHASH,
the current implementation expands the 16 byte hash key
(H) into a table of 64 KBytes. However, that is sensitive
to cache-based timing attacks.

If we assume that access to data inside the same cache line
is constant-time (likely), fitting a table item into a cache
line may help against the attacks.

This patch reduce the pre-computed table from 64K to 4K
and aligns every item to a 32 byte boundary (since most modern
CPUs have cache line of that size or larger).

This patch will reduce the overall performance.

This patch also reverts commit 965871a727 ("GCM mode:
Optimize key setup for GCM mode") since I actually
got conflicting benchmark results.
Make Cipher.galois module private
Realign V tables dynamically