Extends the x86_64 ChaCha20 implementation by a function processing eight
ChaCha20 blocks in parallel using AVX2.

For large messages, throughput increases by ~55-70% compared to four block
SSSE3:

testing speed of chacha20 (chacha20-simd) encryption
test 0 (256 bit key, 16 byte blocks): 42249230 operations in 10 seconds (675987680 bytes)
test 1 (256 bit key, 64 byte blocks): 46441641 operations in 10 seconds (2972265024 bytes)
test 2 (256 bit key, 256 byte blocks): 33028112 operations in 10 seconds (8455196672 bytes)
test 3 (256 bit key, 1024 byte blocks): 11568759 operations in 10 seconds (11846409216 bytes)
test 4 (256 bit key, 8192 byte blocks): 1448761 operations in 10 seconds (11868250112 bytes)

testing speed of chacha20 (chacha20-simd) encryption
test 0 (256 bit key, 16 byte blocks): 41999675 operations in 10 seconds (671994800 bytes)
test 1 (256 bit key, 64 byte blocks): 45805908 operations in 10 seconds (2931578112 bytes)
test 2 (256 bit key, 256 byte blocks): 32814947 operations in 10 seconds (8400626432 bytes)
test 3 (256 bit key, 1024 byte blocks): 19777167 operations in 10 seconds (20251819008 bytes)
test 4 (256 bit key, 8192 byte blocks): 2279321 operations in 10 seconds (18672197632 bytes)

Benchmark results from a Core i5-4670T.
Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>