crypto: x86/serpent-sse2 - convert to skcipher interface

Convert the SSE2 implementation of Serpent from the (deprecated)
ablkcipher and blkcipher interfaces over to the skcipher interface.
Note that this includes replacing the use of ablk_helper with
crypto_simd.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>