Enable AVR as a Tier 3 target upstream
Tracking issue: #44052.
Things intentionally left out of the initial upstream:
* The `target_cpu` flag
I have made the cleanup suggestions by @jplatte and @jplatte in 043550d9db.
Anybody feel free to give the branch a test and see how it fares, or make suggestions on the code patch itself.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.