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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

I did this in https://github.com/llvm/llvm-project/pull/82513 as well, but it adds the extra runtimes things. Don't know if that's the best way to express it, but I'd really prefer to not need to force people to change the rest of their build to enable the cross compiling targets.

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