cmake: toolchain: Fix 'host' toolchain variant
cmake: toolchain: Fix 'host' toolchain variant

Fix an issue with 'ZEPHYR_TOOLCHAIN_VARIANT=host' where CMAKE_C_FLAGS
was incorrectly assumed to be set.

This fixes #21614

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr