[libc] [llvm] [libc][cmake][linux] require new LLVM_LIBC_USE_HOST_KERNEL_HEADERS or LIBC_KERNEL_HEADERS (PR #123820)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> precommit errors:
> 
> ```
> CMake Error at /home/runner/work/llvm-project/llvm-project/libc/cmake/modules/LLVMLibCArchitectures.cmake:221 (message):
>   MUST specify either LIBC_KERNEL_HEADERS or LIBC_USE_HOST_KERNEL_HEADERS
> ```

Ah! Forgot to drop the `LLVM_` prefix in the .github changes!

https://github.com/llvm/llvm-project/pull/123820