sha256 speed

Hello,

On my Ryzen the sha256 command is much slower than openssl dgst -sha256.
For large files, openssl is more than 7 times faster in practice.

You can also test it with the builtin benchmarks:
sha256 -t
openssl speed sha256

I think the reason is that openssl supports the SHA CPU extensions
whereas libmd (used by sha256) does not.

Any chance we can make the base sha256 faster?
I guess there is some reason why we use libmd instead of openssl.

https://reviews.freebsd.org/D2651 looks related but not sure it's still
relevant.