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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> @mstorsjo What files are currently produced? Is there a libLLVM.dll.18.1 file ?

Currently, before this change, the build produces `bin/libLLVM.dll` and `lib/libLLVM.dll.a`, and on install it adds `bin/libLLVM-19git.dll` as a symlink to `bin/libLLVM.dll`. (And currently after this change, the build fails at the cmake stage.)

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