[libc] [llvm] [LLVM] Replace use of `LLVM_RUNTIMES_TARGET` with `LLVM_DEFAULT_TARGET_TRIPLE` (PR #136208)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

This made things unhappy, I think because the LLVM default triple is always defined while the previous one was only defined for these runtime crossbuilds? We could make a new variable and keep them distinct or something.

https://github.com/llvm/llvm-project/pull/136208