Add more optimized XTS code for aesni_intel in 64-bit mode, for smaller stack
usage and boost for speed.

tcrypt results, with Intel i5-2450M:
256-bit key
        enc     dec
16B     0.98x   0.99x
64B     0.64x   0.63x
256B    1.29x   1.32x
1024B   1.54x   1.58x
8192B   1.57x   1.60x

512-bit key
        enc     dec
16B     0.98x   0.99x
64B     0.60x   0.59x
256B    1.24x   1.25x
1024B   1.39x   1.42x
8192B   1.38x   1.42x

I chose not to optimize smaller than block size of 256 bytes, since XTS is
practically always used with data blocks of size 512 bytes. This is why
performance is reduced in tcrypt for 64 byte long blocks.

Cc: Huang Ying <ying.huang@intel.com>
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>