buildroot

target-g++: fix build

Just as we did to fix target-gcc, pass CXX_FOR_TARGET when building
target g++, and remove useless copies of g++ and c++.

Tested on ARM by compiling a simple C++ program using <iostream> on
the target and running it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>