ci: build native_sim with llvm

Author: nashif

.github/workflows/twister.yaml

@@ -135,7 +135,7 @@ jobs:
         run: |
           export ZEPHYR_BASE=${PWD}
           export ZEPHYR_TOOLCHAIN_VARIANT=zephyr
-          ./scripts/twister -G --subset ${{matrix.subset}}/${{ strategy.job-total }} ${TWISTER_COMMON} ${PUSH_OPTIONS}
+          ./scripts/twister -G --force-platform-toolchain native_sim/native:llvm --subset ${{matrix.subset}}/${{ strategy.job-total }} ${TWISTER_COMMON} ${PUSH_OPTIONS}
           if [ "${{matrix.subset}}" = "1" ]; then
             ./scripts/zephyr_module.py --twister-out module_tests.args
             if [ -s module_tests.args ]; then