crypto: lib - move __crypto_xor into utils

CRYPTO_LIB_CHACHA depends on CRYPTO for __crypto_xor, defined in
crypto/algapi.c.  This is a layering violation because the dependencies
should only go in the other direction (crypto/ => lib/crypto/).  Also
the correct dependency would be CRYPTO_ALGAPI, not CRYPTO.  Fix this by
moving __crypto_xor into the utils module in lib/crypto/.

Note that CRYPTO_LIB_CHACHA_GENERIC selected XOR_BLOCKS, which is
unrelated and unnecessary.  It was perhaps thought that XOR_BLOCKS was
needed for __crypto_xor, but that's not the case.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>