gosthash2012_ref: Optimize XLPS for speed

This results in ~20% improvement for reference implementation on my
Xeon E5630.

`openssl speed -evp streebog256' results (before and after):

   16 bytes    64 bytes   256 bytes  1024 bytes  8192 bytes  16384 bytes
 3815.25k   12874.41k   31497.81k   49823.06k   59932.67k    57021.78k
 4361.29k   14933.06k   37091.67k   59001.86k   71360.51k    72488.28k