As far as I know, `libc` is available for every target now. Especially
since the introduction of `bssl::Result` we hardly reference these
types, other than `size_t`. This will help get rid of crypto/crypto.c.
Rename some GCM assembly functions so that all functions that do the
same thing the same way have the same name, to make the dispatching
logic simpler.
Thread CPU feature caching witnesses through the GCM dispatching logic
to make feature detection less error-prone.
Start an internal Rust API for feature detection.
This reverts commit c1c9c2221ef26c7cb808ffe79dad4217bd7f431b.
I am looking at improving the API in a different way, so I'm undoing
this change to reduce churn in the interim.
