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

[Chris B via llvm-commits]

llvm-beanz wrote:

I think this is probably fine. I think @petrhosek's team is probably the most impacted here, and it has been a few years since I was too involved in this area. From looking at the Fuchsia in-tree CMake cache scripts it looks like they explicitly set both builtin and runtime configuration values so this shouldn't break anything.

@petrhosek any thoughts?

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