Rust doesn't support as many CPU architectures as C does (SH4 for example, though there's likely many more better examples.)

This might make a much more interesting case for GOT than before https://www.gameoftrees.org/

My understanding: As Rust is built on LLVM and not GCC, it is also limited to operating systems supporting LLVM.

GCC simply supports more platforms.

because the rust compiler just doesn't support some platforms (os / architecture combination)?

RESF members tend to say it the other way around as in the platform doesn't support rust, but the reality is that it's the compiler that needs to support a platform, not the other way around.

See this page [1], particularly the 'Tier 3' platforms.

[1] https://doc.rust-lang.org/beta/rustc/platform-support.html