From the blog post, it sounds like the problem is that Rust doesn't use standard LLVM, they maintain a separate branch with a bunch of patches that are specific to Rust. So, adding AVR support to Rust means adding AVR support to Rust's branch of LLVM, which is a hassle. Rust is about to move to a newer version of LLVM, though, so when that happens they'll be pulling in the AVR support that's already in mainline LLVM.