crypto: x86/chacha20 - Support partial lengths in 4-block SSSE3 variant

Add a length argument to the quad block function for SSSE3, so the
block function may XOR only a partial length of four blocks.

As we already have the stack set up, the partial XORing does not need
to. This gives a slightly different function trailer, so we keep that
separate from the 1-block function.

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