build native_sim with llvm

Author: nashif

scripts/pylib/twister/twisterlib/runner.py

@@ -616,8 +616,10 @@ def run_cmake(self, args="", filter_stages=[]):
             f'-D{warning_command}={warnings_as_errors}',
             f'-DEXTRA_GEN_EDT_ARGS={gen_edt_args}',
             f'-G{self.env.generator}',
-            f'-DPython3_EXECUTABLE={pathlib.Path(sys.executable).as_posix()}'
+            f'-DPython3_EXECUTABLE={pathlib.Path(sys.executable).as_posix()}',
         ]
+        if self.platform.name == 'native_sim/native':
+            cmake_args.append(f'-DZEPHYR_TOOLCHAIN_VARIANT=llvm')
 
         # If needed, run CMake using the package_helper script first, to only run
         # a subset of all cmake modules. This output will be used to filter