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

[via llvm-commits]

lntue 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
```

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