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

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

I think that @petrhosek and @llvm-beanz definitely should have a look over this - they have been working on the runtimes build, and would have a better idea of how they would like to see it evolve.

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