crypto: lib - create utils module and move __crypto_memneq into it

As requested at
https://lore.kernel.org/r/YtEgzHuuMts0YBCz@gondor.apana.org.au

, move
__crypto_memneq into lib/crypto/ and put it under a new tristate.  The
tristate is CRYPTO_LIB_UTILS, and it builds a module libcryptoutils.  As
more crypto library utilities are being added, this creates a single
place for them to go without cluttering up the main lib directory.

The module's main file will be lib/crypto/utils.c.  However, leave
memneq.c as its own file because of its nonstandard license.

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>