samples: syst: exclude XCC from C++ tests
samples: syst: exclude XCC from C++ tests

The logging subsys has evolved to a point where XCC/GCC cannot
compile anymore as it only supports C++98. So exclude C++
samples from build. And just use XCC/Clang instead for C++.

Signed-off-by: Daniel Leung <d94e04d205b5962d7873a139ef298b03c1717f72@intel.com>

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