[llvm] [CMake] Fix DynamicLibraryTests exports symbol when plugins are enabled (PR #102941)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

I reverted. @jakeegan @qiongsiwu I might need some help to figure out why currently it fails on AIX to figure out a fix.

Specifically, what is the difference made by https://github.com/llvm/llvm-project/pull/102671 this patch? Does it generate a different export file on AIX and how is it different?

That might also help me to understand how the PR mentioned above can break seemingly unrelated tests. 

https://github.com/llvm/llvm-project/pull/102941