crypto: x86/serpent- switch to XTS template

Now that the XTS template can wrap accelerated ECB modes, it can be
used to implement Serpent in XTS mode as well, which turns out to
be at least as fast, and sometimes even faster

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