[llvm] [llvm][CMake] Introduce LLVM_RUNTIME_<project>_BUILD in CMake (PR #88934)

[Petr Hosek via llvm-commits]

petrhosek wrote:

> I have moved the code to runtimes/CMakeLists.txt now. Does that look better now?

It should be moved to `runtimes/CMakeLists.txt`, not `llvm/runtimes/CMakeLists.txt`.

https://github.com/llvm/llvm-project/pull/88934