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

I was trying to run the tests on FreeBSD and noticed that we weren't
printing symbol names. It turns out this is because of the missing
-Wl,--export-dynamic flag. Instead of hardcoding the name of the flag
and only passing it for Linux hosts, use a pre-existing CMake variable
instead. I was not aware of this flag, but it appears to have been
supported for the past 16 years (with support for more platforms added
later):
https://gitlab.kitware.com/cmake/cmake/-/commit/66d1930f5674f08e09f455b3f0777f2de3e0717e