chapoly: Process two Poly1305 blocks in parallel in SSSE3 driver

chapoly: Process two Poly1305 blocks in parallel in SSSE3 driver

By using a derived key r^2 we can improve performance, as we can do loop
unrolling and slightly better utilize SIMD instructions.

Overall ChaCha20-Poly1305 performance increases by ~12%.

Converting integers to/from our 5-word representation in SSE does not seem
to pay off, so we work on individual words.