crypto: x86/chacha20 - Support partial lengths in 1-block SSSE3 variant

Add a length argument to the single block function for SSSE3, so the
block function may XOR only a partial length of the full block. Given
that the setup code is rather cheap, the function does not process more
than one block; this allows us to keep the block function selection in
the C glue code.

The required branching does not negatively affect performance for full
block sizes. The partial XORing uses simple "rep movsb" to copy the
data before and after doing XOR in SSE. This is rather efficient on
modern processors; movsw can be slightly faster, but the additional
complexity is probably not worth it.

Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>