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

[Shoaib Meenai via llvm-commits]

smeenai wrote:

Looks useful :) 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? (That'd also allow you to have a common function that you just call from both `llvm_add_executable` and `llvm_add_library` instead of duplicating any logic.)

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