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

 #!/bin/sh

 # Needed for systems without gettext

 $* -xc -o /dev/null - > /dev/null 2>&1 << EOF

 #include <libintl.h>

 int main()

 {

 	gettext("");

 	return 0;

 }

 EOF

 if [ ! "$?" -eq "0"  ]; then

 	echo -DKBUILD_NO_NLS;

 fi