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

[Chris B via llvm-commits]

llvm-beanz wrote:

@jhuber6 the reason I initially implemented builtins and runtimes separately is because the runtime configuration completing successfully and accurately depends on the builtin libraries being available during configuration. We could probably make some educated guess hard-coded assumptions about what the toolchain supports, but you can quickly get off the rails with more exotic target support.

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