Import SHA-3 code into libc and libkern.

No new public symbols in libc, but publishing the symbols is a simple
matter if/when we decide to do so.

Proposed on tech-kern and tech-userlevel with no objections:

https://mail-index.NetBSD.org/tech-kern/2017/11/11/msg022581.html
https://mail-index.NetBSD.org/tech-userlevel/2017/11/11/msg010968.html