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

Then standard cargo tool may be used to directly build 6502 executable, some examples: https://github.com/mrk-its/a800-rust-test or https://github.com/mrk-its/llvm-mos-ferris-demo