crypto: xctr - Add XCTR support

Add a generic implementation of XCTR mode as a template. XCTR is a
blockcipher mode similar to CTR mode. XCTR uses XORs and little-endian
addition rather than big-endian arithmetic which has two advantages: It
is slightly faster on little-endian CPUs and it is less likely to be
implemented incorrect since integer overflows are not possible on
practical input sizes. XCTR is used as a component to implement HCTR2.

More information on XCTR mode can be found in the HCTR2 paper:
https://eprint.iacr.org/2021/1441.pdf

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