LLVM is a much nicer for cross compilation. It has been multi-target for the vast majority, and its associated runtime libraries (e.g. compiler-rt, libcxx) are readily built separately from the compiler itself.

Unfortunately, our current packaging doesn't take advantage of either of these. We build compiler-rt in the same derivation as LLVM, and we pull everything from the same boostrappping stage, precluding cross compilation infra from working its magic.

I took a stab at this in obsidiansystems@acf2ed5 (the top commit in https://github.com/obsidiansystems/nixpkgs/tree/split-compiler-rt). I should get around to at least rebasing that, but even in its current state that serves as a basic reference to what should be accomplished.

CC @angerman @dtzWill