[llvm] [LLVM][runtimes] Prepopulate `LLVM_BUILTIN_TARGETS` with runtimes values (PR #95554)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

Also, @petrhosek, what's the specific reason we need to build these in a separate CMake invocation? Doesn't the list of runtimes have an implicit order which we build them? I think we could add a dependency for the `builtins` target within a single CMake invocation if needed. I'm wondering because adding these as separate targets creates a significant overhead to rebuild an extra CMake project.

https://github.com/llvm/llvm-project/pull/95554