Change twofish-avx to use the new XTS code, for smaller stack usage and small
boost to performance.

tcrypt results, with Intel i5-2450M:
        enc     dec
16B     1.03x   1.02x
64B     0.91x   0.91x
256B    1.10x   1.09x
1024B   1.12x   1.11x
8192B   1.12x   1.11x

Since XTS is practically always used with data blocks of size 512 bytes or
more, I chose to not make use of twofish-3way for block sized smaller than
128 bytes. This causes slower result in tcrypt for 64 bytes.
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>