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

[Alexander Richardson via cfe-commits]

arichardson wrote:

> Can you as a small follow-up just update the comment to include that part? E.g.
> 
Will do.

> ```
> # 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`.

It appears CMake actually still does this in one case:
```
Modules/Platform/Linux-TinyCC-C.cmake:set(CMAKE_EXE_EXPORTS_C_FLAG "-rdynamic ")
```

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