gosthash2012: Remove temporary variable from `stage2'

Now `stage2' will always get aligned data which is prepared in
`gost2012_hash_block' by copying into `CTX->buffer'.

This will allow to change `data' argument of `stage2' from `unsigned
char *' pointer to `union uint512_u *'.

Committed-by: Vitaly Chikunov <vt@altlinux.org>