cmake: clang: Allow host's clang to be used for native_posix
cmake: clang: Allow host's clang to be used for native_posix

Allow host installed clang to be used for native_posix when
ZEPHYR_TOOLCHAIN_VARIANT=llvm.

Signed-off-by: Oleg Zhurakivskyy <cdb82bfa180f8df716a77d2c2557196a597a233c@intel.com>

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