Author of mentioned post on 6502.org forum here. In the meantime I worked a bit on implementing proper rust target-triple for 6502 (mos-unknown-none), code is here: https://github.com/mrk-its/rust/tree/mos_target
That's cool! I wanted to avoid having to build Rust and/or LLVM from source myself, hence the somewhat awkward "tell Cargo we're on default target, let Clang sort it out at link time" setup.