Assert at run time instead. Fixes #13756.

I’d rather have this be detected at compile-time, but I don’t know how to do that.

Fix by @Gankro!

To make progress on #18585 we're paring down the distribution to theoretically
"only libstd", and this commit makes progress on this by removing the rlibc
crate from the distribution.

The crate has now been moved into an external cargo package located in the rust
lang organization [1]. This is a breaking change due to this removal, and
existing crates depending on `rlibc` should use the Cargo crate instead.

[1]: https://github.com/rust-lang/rlibc

[breaking-change]
cc #18585

Right now the windows nightlies are failing because they're encountering a
linker error when producing stage3 libs. The stage3 libs aren't actually used in
general, and we primarily just want to generate a static stage3 binary, not
static stage3 dylibs.

As a new user, I spent a while confused when flycheck told me the code sample I'd typed in was invalid. I ended up figuring out some of what comes after the code sample more painfully by myself because there was no indication that it was broken in the text beforehand. This one line change makes it clear that the code following it is an experiment that may not work rather than something to assume just works.

- ToBase64
- FromBase64
- ToHex
- FromHex
- ToJson
- Encodable

Fixes #17023