crypto: chacha20 - Add a SSSE3 SIMD variant for x86_64

Implements an x86_64 assembler driver for the ChaCha20 stream cipher. This
single block variant works on a single state matrix using SSE instructions.
It requires SSSE3 due the use of pshufb for efficient 8/16-bit rotate
operations.

For large messages, throughput increases by ~65% compared to
chacha20-generic:

testing speed of chacha20 (chacha20-generic) encryption
test 0 (256 bit key, 16 byte blocks): 45089207 operations in 10 seconds (721427312 bytes)
test 1 (256 bit key, 64 byte blocks): 43839521 operations in 10 seconds (2805729344 bytes)
test 2 (256 bit key, 256 byte blocks): 12702056 operations in 10 seconds (3251726336 bytes)
test 3 (256 bit key, 1024 byte blocks): 3371173 operations in 10 seconds (3452081152 bytes)
test 4 (256 bit key, 8192 byte blocks): 422468 operations in 10 seconds (3460857856 bytes)

testing speed of chacha20 (chacha20-simd) encryption
test 0 (256 bit key, 16 byte blocks): 43141886 operations in 10 seconds (690270176 bytes)
test 1 (256 bit key, 64 byte blocks): 46845874 operations in 10 seconds (2998135936 bytes)
test 2 (256 bit key, 256 byte blocks): 18458512 operations in 10 seconds (4725379072 bytes)
test 3 (256 bit key, 1024 byte blocks): 5360533 operations in 10 seconds (5489185792 bytes)
test 4 (256 bit key, 8192 byte blocks): 692846 operations in 10 seconds (5675794432 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>