Fix building several modules as built-in

[ojeda]

Currently building several built-in modules does not work since shared crates are re-compiled (and therefore re-linked) for each module.

[ojeda]

To expand a bit on this: a quick workaround would be hiding symbols, but ideally we want to share the common code, even for loadable modules. The reason is that we want to be able to build kernels with Rust support built-in so that Rust loadable modules can be as small as possible.

In fact, that will likely be the most common use case as soon as distro kernels start shipping with Rust modules. Until then, third-party modules would still need a way to embed the Rust support.