*ring* defined a function named `OPENSSL_memcmp` that did what
`CRYPTO_memcmp` does in BoringSSL, and BoringSSL has a different
function called `OPENSSL_memcmp`. *ring* doesn't need
`OPENSSL_memcmp` so sync the `CRYPTO_memcmp` stuff with BoringSSL.
This eliminates unnecessary differences from BoringSSL.