[libc] [llvm] [build] Check RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES for libc also (PR #82561)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 approved this pull request.

I'll approve this to move along whatever you have planned. This should allow using it with the existing support, however I strongly think that we need an "append only" solution if that makes sense. I want a solution that only adds CMake arguments so users can just copy-paste something or use some shorthand that does the same thing.

https://github.com/llvm/llvm-project/pull/82561