Rollup merge of #134659 - jieyouxu:recursive-remove, r=ChrisDenton

test-infra: improve compiletest and run-make-support symlink handling

I was trying to implement #134656 to port `tests/run-make/incr-add-rust-src-component.rs`, but found some blockers related to symlink handling, so in this PR I tried to resolve them by improving symlink handling in compiletest and run-make-support (particularly for native windows msvc environment).

Key changes:

- I needed to copy symlinks (duplicate a symlink pointing to the same file), so I pulled out the copy symlink logic and re-exposed it as `run_make_support::rfs::copy_symlink`. This helper correctly accounts for the Windows symlink-to-file vs symlink-to-dir distinction (hereafter "Windows symlinks") when copying symlinks.
- `recursive_remove`:
    - I needed a way to remove symlinks themselves (no symlink traversal). `std::fs::remove_dir_all` handles them, but only if the root path is a directory. So I wrapped `std::fs::remove_dir_all` to also handle when the root path is a non-directory entity (e.g. file or symlink). Again, this properly accounts for Windows symlinks.
    - I wanted to use this for both compiletest and run-make-support, so I put the implementation and accompanying tests in `build_helper`.
    - In this sense, it's a reland of #129302 with proper test coverage.
    - It's a thin wrapper around `std::fs::remove_dir_all` (`remove_dir_all` correctly handles read-only entries on Windows). The helper has additional permission-setting logic for when the root path is a non-dir entry on Windows to handle read-only non-dir entry.

Fixes #126334.

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