tests: cpp: libcxx: Use full version of newlib for testing
tests: cpp: libcxx: Use full version of newlib for testing

Use the full version of newlib (i.e. not nano) for libcxx testing since
some features (e.g. C++ exception handling) require the full version of
newlib.

Signed-off-by: Stephanos Ioannidis <dc76e9f0c0006e8f919e0c515c66dbba3982f785@stephanos.io>

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