[llvm] [Runtimes] Pass through per-runtime CMake options for target runtimes (PR #194105)

[Wenju He via llvm-commits]

wenju-he wrote:

> explain what's the issue you're trying to address so we can advise on a correct solution?

The issue is that the runtime prefix isn't passed through when using a per-target runtime build (e.g., via `-DRUNTIMES_triple-_LLVM_ENABLE_RUNTIMES=libclc -DLLVM_RUNTIME_TARGETS="triple"` ) instead of `-DLLVM_ENABLE_RUNTIMES`.

> After this change we would pass all runtime prefixes to all per-target runtime builds by default which can have undesirable consequence

Ideally the intention was to route a runtime prefix only to its own specific build, not to all runtimes.

https://github.com/llvm/llvm-project/pull/194105