[llvm] [CMake] Support per-target linker flags (PR #68393)

[Vincent Lee via llvm-commits]

thevinster wrote:

@petrhosek Does this change look reasonable to you? I know this particular issue could be solved with `-ffat-lto-objects`, but I think this could apply to other places as well. 

https://github.com/llvm/llvm-project/pull/68393