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

[Fangrui Song via llvm-commits]

MaskRay wrote:

I have wanted this for a long time.  Thank you for the patch. (I am not familiar with CMake.) Does this variable need to be cached? New CMake variables can be documented at llvm/docs/CMake.rst

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