tests: Port `extern-fn-reachable` to rmake.rs
Part of #121876.
## Summary
This PR ports `tests/run-make/extern-fn-reachable` to use `rmake.rs`. Notable changes:
- We now use the `object` crate and look at the exported symbols specifically.
- This test's coverage regressed against windows-msvc back in [replace dynamic library module with libloading #90716](https://github.com/rust-lang/rust/pull/90716), but since we use `object` now, we're able to claw the test coverage back.
- The checks are now stricter:
1. It no longer looks for substring symbol matches in `nm` textual outputs, it inspects the symbol names precisely.
2. We now also explicitly check for the presence of leading underscore in exported symbol names on apple vs non-apple targets.
- Added another case of `#[no_mangle] fn fun6() {}` (note the lack of `pub`) to check that Rust nameres visibility is orthogonal to symbol visibility in dylib.
## History
- Test was initially introduced as a run-pass[^run-pass] test as part of [Don't mark reachable extern fns as internal #10539](https://github.com/rust-lang/rust/pull/10539).
- Test re-introduced as a run-make test in https://github.com/rust-lang/rust/pull/13741.
- Later, the test coverage regressed in https://github.com/rust-lang/rust/pull/90716.
[^run-pass]: no longer a thing nowadays
Supersedes #128314.
Co-authored with `@lolbinarycat.`
try-job: x86_64-msvc
try-job: i686-msvc
try-job: i686-mingw
try-job: x86_64-mingw-1
try-job: x86_64-apple-1
try-job: aarch64-apple
try-job: test-various
5cd16b7f2b