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

[Vincent Lee via llvm-commits]

thevinster wrote:

> Do we need the granularity of the exe vs. module vs. shared custom flags, or could we just have a single set of custom flags per target? 

Initially, I separated because I thought the flags would've been duplicated, but I now realize that the two methods are mutually exclusive (i.e. `clang` target would not be invoked with `llvm_add_library` and same goes for a library being invoked with `llvm_add_executable`)

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