[libunwind] [libunwind] Pass -Wl,--export-dynamic on all supported platforms (PR #67205)

[Joerg Sonnenberger via cfe-commits]

jsonn wrote:

Can you as a small follow-up just update the comment to include that part? E.g.
```
# On ELF platforms, link tests with -Wl,--export-dynamic if supported by the linker.
```

Side note: the much older way for doing this is actually `-rdynamic`.

https://github.com/llvm/llvm-project/pull/67205