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

[Michael Klemm via llvm-commits]

mjklemm wrote:

Thanks for taking the time to review and the explanation about the mechanics!

> This patch only sets `LLVM_RUNTIME_<project>_BUILD` in the _bootstrapping build_ which is not the right place, it should be set in the _runtimes build_ (that is https://github.com/llvm/llvm-project/blob/main/runtimes/CMakeLists.txt) to make sure that the variable is set whether we're using runtimes or bootstrapping build.

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

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