[llvm] [CMake] Enable per-target runtime directory by default (PR #200881)

[Petr Hosek via llvm-commits]

petrhosek wrote:

As I mentioned in https://github.com/llvm/llvm-project/pull/200269#issuecomment-4595041870, we've been using `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` on Windows for years and we never encountered any issues, but I can also send a short RFC and ask if anyone is aware of any blockers.

https://github.com/llvm/llvm-project/pull/200881