cc/gcc: Set CXX_FOR_BUILD for bare metal and canadian build.

>From 4.8, g++ is used as the default compiler to build the toolchain.

Signed-off-by: Zhenqiang Chen <zhenqiang.chen@linaro.org>
Message-Id: <CACgzC7B-LQvAw3hOYhBA7b7g0H1WtH20gqXM=Y=YFO4FrnZKWQ@mail.gmail.com>
Patchwork-Id: 243590