Auto merge of #9566 - ehuss:relative-rustc-path, r=alexcrichton
Fix rustc/rustdoc config values to be config-relative.
The `rustc`, `rustdoc`, `rustc_wrapper`, and `rustc_workspace_wrapper` config values (in the `[build]` table) were being interpreted literally. This caused a problem if you used a relative path like `foo/rustc`. This would be interpreted as a relative path from whatever cwd cargo launches rustc from, which changes for different scenarios, making it essentially unusuable (since crates.io dependencies wouldn't be buildable).
Additionally, due to https://github.com/rust-lang/rust/issues/37868, it is a bad idea to use relative paths.
This changes it so that those paths are config-relative. Bare names (like "my-rustc-program") still use PATH as before.
This also includes a commit to centralize the rustc-wrapper program used by several tests so that it isn't built multiple times (and to allow several tests to work on windows).