Auto merge of #113341 - Kobzol:stage0-sysroot, r=Mark-Simulacrum

Copy stage0 `rustc` binaries to `stage0-sysroot`

This is basically a revival of rust-lang/rust#101711 and rust-lang/rust#107956, with an added check that the full sysroot will only be created if the original rustc comes from `stage0/bin`.

What is/should be tested:

- [x] `rustup toolchain link stage0` (new libstd is used correctly)
- [x]  `python3 x.py fmt dist --stage 0`
- [x] Custom rustc/cargo in `config.toml` (in this case this logic is ignored)
- [x]  Perfbot (try perf run has succeeded)
- [x] Real use case (rust-lang/backtrace-rs#542)

(Hopefully) fixes: rust-lang/rust#101691

This is not the "end all, be all" solution to this problem, but as long as it resolves the basic use-case, and doesn't break perfbot, I say ship it. This code will probably be nuked anyway Soon™ because of the stage redesign.