Create a generic AVR target: avr-none

This commit removes the `avr-unknown-gnu-atmega328` target and replaces
it with a more generic `avr-none` variant that must be specialized with
the `-C target-cpu` flag (e.g. `-C target-cpu=atmega328p`).

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1990,6 +1990,7 @@ fn add_pre_link_args(cmd: &mut dyn Linker, sess: &Session, flavor: LinkerFlavor)
     if let Some(args) = sess.target.pre_link_args.get(&flavor) {
         cmd.verbatim_args(args.iter().map(Deref::deref));
     }
+
     cmd.verbatim_args(&sess.opts.unstable_opts.pre_link_args);
 }