bootstrap: Print more debug info when find_initial_libdir fails

[Zalathar]

From looking at the failure messages printed by #129775, my hypothesis is that rustc --print=sysroot sometimes prints the wrong path when the rustc executable is hardlinked in multiple places, at least on my macOS system.

However, currently I don't have any concrete evidence of this actually happening. This PR therefore expands on #129775 by printing even more information on failure (including the actual rustc path), in the hope that when the failure next occurs we can confirm or reject the hypothesis that rustc --print=sysroot is printing the wrong path.

[rustbot]

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[Zalathar]

My hypothesis is based on the following:

• From looking at how bootstrap determines the paths involved, I can't think of any reason why it would be trying to run the stage0-sysroot compiler instead of the stage0 compiler.
• The stage0-sysroot rustc and stage0 rustc are both hard links to the same underlying file.
• The compiler normally uses dladdr to determine its own path (to find the corresponding sysroot).
• https://jmmv.dev/2020/01/osxfuse-hardlinks-dladdr.html suggests that on APFS, dladdr can potentially return the “wrong” path in the presence of hard links.

This is still very speculative, which is why I'm trying to gather more information before taking action.

[albertlarsan68]

LGTM

[albertlarsan68]

@bors r+ rollup

[bors]

📌 Commit a21f395 has been approved by albertlarsan68

It is now in the queue for this repository.

[Zalathar]

Finally witnessed this failing again on my machine:

Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
thread 'main' panicked at src/lib.rs:348:13:
couldn't determine `initial_libdir`:
- config.initial_rustc:      "/Users/stuart/Dev/rust/rust/build/aarch64-apple-darwin/stage0/bin/rustc"
- initial_target_libdir_str: "/Users/stuart/Dev/rust/rust/build/aarch64-apple-darwin/stage0-sysroot/lib/rustlib/aarch64-apple-darwin/lib\n"
- initial_target_dir:        "/Users/stuart/Dev/rust/rust/build/aarch64-apple-darwin/stage0-sysroot/lib/rustlib/aarch64-apple-darwin"
- initial_sysroot:           "/Users/stuart/Dev/rust/rust/build/aarch64-apple-darwin/stage0"

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Build completed unsuccessfully in 0:00:00