Right now, we pick the target triples to pass clang by defaulting to the first segment of the target triple and then manually fixing them up with a large set of branches. Is there a way we could query the target spec from rustc to get the LLVM target names? For example, by running:

$ rustc +nightly -Zunstable-options --print all-target-specs-json 

We can obtain a JSON dump of all of the target configurations. Each target configuration contains an "llvm-target" field which we should be passing to clang. That's not stable right now, but maybe there's some other way to work with that?