Rollup merge of #134829 - jieyouxu:migrate-libs-through-symlinks, r=lqd

Migrate `libs-through-symlink` to rmake.rs

Part of https://github.com/rust-lang/rust/issues/121876.

This PR migrates `tests/run-make/libs-through-symlink/` to use rmake.rs.

- Regression test for #13890.
- Original fix PR is #13903.
- Document test intent, backlink to #13890 and fix PR #13903.
- Fix the test logic: the `Makefile` version seems to not actually be exercising the "library search traverses symlink" logic, because the actual symlinked-to-library is present under the `$(TMPDIR)` directory tree when `bar.rs` is compiled, because the `$(RUSTC)` invocation has an implicit `-L $(TMPDIR)`. The symlink itself was actually broken, i.e. it should've been `ln -nsf $(TMPDIR)/outdir/$(NAME) $(TMPDIR)` but it used `ln -nsf outdir/$(NAME) $(TMPDIR)`. The rmake.rs version now explicitly separates the two directory trees and sets the CWD of the `bar.rs` rustc invocation so that the actual library is *not* present under its CWD tree.

I.e. it is now

```
$test_output/           # rustc foo.rs -o actual_lib_dir/libfoo.rlib
    actual_lib_dir/
        libfoo.rlib
    symlink_lib_dir/    # CWD set; rustc -L . bar.rs
        libfoo.rlib --> $test_output/actual_lib_dir/libfoo.rlib
```

Partially supersedes #129011.
This PR is co-authored with `@Oneirical.`

r? compiler

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