@k0nze @thomholwerda I can only speak for SPARC since that is what I use, but yeah, native build servers do exist and are maintained by volunteers.
There's a couple maintained by the Compile Farm project[1] (in which you can request SSH access to experiment with porting, etc.) and LLVM also has a buildbot machine for its CI[2].

As far as Rust is concerned, SPARC (at least the 64-bit variant) platforms are listed as official targets in rustc documentation[3]. Of course, yes, it's only at tier 2/3 (unfortunately!) but it's still very functional - at the very least Gentoo and OpenBSD[4] ships with some selection of Rust packages in their repos, and rustc does work okay over there.
Though, performance-wise it (and other compilers implemented on top of LLVM - including clang itself) still generates slllliiiiiightly slower code than GCC because atm LLVM's SPARC backend isn't as aggressively optimizing as its GCC counterpart, but this is being actively worked on too.

For more general issue tracker, there's also the sparclinux project who provides it[5].

In short, using Rust or other modern tools are very possible on a SPARC box, but clearly there's still a lot more to do so any help here would be very welcome  
Guess tagging this with #rust #sparc #sparc64 too for visibility haha

1: https://portal.cfarm.net/machines/list/
2: https://lab.llvm.org/staging/#/builders/82
3: https://doc.rust-lang.org/nightly/rustc/platform-support.html
4: It's unfortunate too that Solaris support require Solaris 11.4 upwards, which I think rules out the vast majority of hobbyist machines (since 11.4 requires SPARC T4 or SPARC64 X processor from ~2011 as a baseline), but this is something that the Solaris porters at LLVM have decided, from what I understand.
5: https://github.com/sparclinux/issues/issues