Cleanup target files

[Rahix]

Notes to myself about what needs to happen:

• Move all target files into a single top-level directory. Update all .cargo/config.tomls to point to the files in this place instead. -> This gets rid of the symlinks thus fixing Windows build fails because of symlinked target files #29.
• Do this cleanup to all our target files as well.
• Check remaining settings against current upstream defaults (after [AVR] Replace broken 'avr-unknown-unknown' target with 'avr-unknown-gnu-atmega328' target rust-lang/rust#74941)
• Think about maybe adding infrastructure for automatically deriving the target files from upstream defaults to reduce maintenance burden.

[explicite]

@Rahix can you please mark second checkbox?

[explicite]

Looks line in third one we have some additional setting which I cannot add find in our targets.
https://github.com/rust-lang/rust/pull/74941/files#diff-e26bdcc4844624b6b557a83e64e20c0a
I cannot see e.g eh_frame_header or max-atomic-width. Should we support all old and new options or remove legacy from our targets? Please find bellow example for all merged.

{
  "arch": "avr",
  "data-layout": "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8",
  "llvm-target": "avr-unknown-unknown",
  "target-endian": "little",
  "target-pointer-width": "16",
  "linker-flavor": "gcc",
  "os": "unknown",
  "target-env": "",
  "target-vendor": "unknown",
  "target-c-int-width": "16",
  "cpu": "atmega2560",
  "exe-suffix": ".elf",
  "linker": "avr-gcc",
  "dynamic_linking": false,
  "executables": true,
  "linker_is_gnu": true,
  "has_rpath": false,
  "position_independent_executables": false,
  "eh_frame_header": false,
  "max-atomic-width": 8,
  "pre-link-args": {
    "gcc": [
      "-mmcu=atmega2560"
    ]
  },
  
  "post-link-args": {
    "gcc": [
      "-Wl,--gc-sections"
    ]
  },
  "singlethread": false,
  "no-builtins": false,
  "no-default-libraries": false,
  "eh-frame-header": false
}

[Rahix]

This command shows the current upstream default target:

rustc --print target-spec-json -Z unstable-options --target avr-unknown-gnu-atmega328

[Rahix]

@explicite, I suggest not spending too much time on this now because as I wrote in the last bullet point, ideally I want to move towards autogeneration of these target files.

[explicite]

@Rahix, maybe I can use https://github.com/serde-rs/json and create some trait with default impl for generic. Then I will override per chip? Then magic can be done in build.rs?

[Rahix]

@explicite, no the target file must exist before invoking cargo so this can't happen in build.rs. I do want to keep the target files checked into the repo for that reason. My idea was this:

• A toplevel directory named avr-specs/ (bikeshedding welcome) containing all the target specs we have.
• There's a small python script named avr-specs/sync-from-upstream.py which runs rustc to retrieve the current upstream spec, patches the few things necessary, and overwrites the files.
• I'll then run this script from time to time to keep our specs up-to-date.