Makefile.toolchain.zephyr: fix C++ on Xtensa
Makefile.toolchain.zephyr: fix C++ on Xtensa

Need to set CXXFLAGS just like we did CFLAGS.

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

bigdinotech/zephyr,bigdinotech/zephyr,bigdinotech/zephyr,bigdinotech/zephyr,bigdinotech/zephyr