Check that Rust config is enabled

As per the discussion on #1890 with @ojeda:

> By the way, this is true if there is no reconfiguring; otherwise, Rust gets disabled automatically. Either way, it may be a good idea to check with make `LLVM=1 rustavailable` and/or inspecting `CONFIG_RUST=y` in the config -- not sure if we are doing something like that already (or if something else will break if Rust got disabled for any reason, e.g. a test that checks one of the Rust samples got built/ran successfully or similar -- we do this kind of thing in our GitHub CI).

So for Rust builds, we could add steps to do those checks with to have a clear error rather than having a build silently completing without Rust support.

Start by tracing the Rust build path and the existing GitHub CI checks; the issue names `make LLVM=1 rustavailable`, `CONFIG_RUST=y`, and Rust samples as the relevant signals. Confirm how a disabled Rust configuration currently behaves, then make the build report a clear error when Rust support is unavailable rather than completing silently.

Written by the indexing model from the issue text.