[llvm] llvm-shlib: Fix libLLVM-${MAJOR}.so symlink on MacOS (PR #85163)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

This broke building the dylib for mingw platforms, see e.g. https://github.com/mstorsjo/llvm-mingw/actions/runs/8303983325/job/22728993479:
```
CMake Error:
  Error evaluating generator expression:

    $<TARGET_SONAME_FILE_NAME:LLVM>

  TARGET_SONAME_FILE is not allowed for DLL target platforms.
```
Could we revert this until we sort it out, to get my builds back to green?


https://github.com/llvm/llvm-project/pull/85163