[llvm] [offload] Fix link issues when LLVM_LINK_LLVM_DYLIB on (PR #106583)

[via llvm-commits]

macurtis-amd wrote:

> Can we fix that then?

At least in the case of PluginCommon, there's [this comment](https://github.com/llvm/llvm-project/blob/fab925651685505906416dca48469fd9f69ba39a/offload/plugins-nextgen/common/CMakeLists.txt#L1-L2):

```
# NOTE: Don't try to build `PluginInterface` using `add_llvm_library` because we
# don't want to export `PluginInterface` while `add_llvm_library` requires that.
```

https://github.com/llvm/llvm-project/pull/106583