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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

I get this error from the bot. Any clue what the intent of this check is...?
```
CMake Error at /home/runner/work/llvm-project/llvm-project/libc/cmake/modules/LLVMLibCArchitectures.cmake:210 (message):
  GCC target triple (x86_64-linux-gnu) and the explicity specified target
  triple (x86_64-unknown-linux-gnu) do not match.
```

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