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

[Alexander Richardson via cfe-commits]

arichardson wrote:

> Is this really desirable? It can actually break applications as it changes ELF semantics.

I agree that it's not generally desirable, but this is only for the libunwind tests (which expect these semantics).

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