Warn about mixed binaries

[polazarus]

Following Exploiting Mixed Binaries

In particular, now that Rustc supports the usual hardenings. It should be done on mixed binaries. At least Rust should not lower the security level of mixed binaries.

[polazarus]

Other leads in isolating the foreign/unsafe code:

• https://www.usenix.org/conference/usenixsecurity23/presentation/bang
• https://www.arxiv.org/abs/2507.03344
• https://dl.acm.org/doi/abs/10.1145/3485832.3485903
• https://www.usenix.org/conference/osdi25/presentation/schuermann