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

 --- gcc-4.3.4/libffi/src/arm/sysv.S	

 +++ gcc-4.3.4/libffi/src/arm/sysv.S	

 @@ -235,7 +235,7 @@ ARM_FUNC_START ffi_closure_SYSV

  	stmfd	sp!, {ip, lr}

  	UNWIND .save	{r0, lr}

  	add	r2, sp, #8

 -	.pad #16

 +	UNWIND .pad #16

  	sub	sp, sp, #16

  	str	sp, [sp, #8]

  	add	r1, sp, #8