@p @niconiconi LLVM/Clang can be built all of it's supported architectures at once (default behavior on gentoo AFAIK), which is pretty great for freestanding executables.
I haven't tried to mess with cross-compiling OS executables with it so far though but I expect it to be nearly just kernel headers + libc of choice without having a bunch of stages to get a compiler for a triple like done with crossdev/buildroot/yocto/…

btw cross-compiling C/C++ stuff is something Zig (LLVM-based) got pretty well known for.