[llvm] [cmake] Forward MacOS sysroot to runtimes when not crosscompiling (PR #182501)

[Michael Kruse via llvm-commits]

================
@@ -4,6 +4,11 @@
 # the two files.
 
 set(COMMON_CMAKE_ARGS "-DHAVE_LLVM_LIT=ON;-DCLANG_RESOURCE_DIR=${CLANG_RESOURCE_DIR}")
+set(NATIVE_RUNTIMES_CMAKE_ARGS)
----------------
Meinersbur wrote:

```suggestion
set(NATIVE_RUNTIMES_CMAKE_ARGS "")
```
`set(VAR)` is equivalent to `unset(VAR)`. Here is the CMake manual recommendation:

<img width="818" height="150" alt="Image" src="https://github.com/user-attachments/assets/bf5067f5-5d98-4436-96d6-d31b0a4d8c38" />


https://github.com/llvm/llvm-project/pull/182501