gcc: preserve CXXFLAGS_FOR_TARGET

gcc: preserve CXXFLAGS_FOR_TARGET

gcc-4.7.x, gcc-4.8.x and gcc-4.9.x don't propagate CXXFLAGS_FOR_TARGET to
CXXFLAGS for libstdc++ build. As a result libstdc++ is built without
TARGET_CFLAGS and may fail to link with applications using it, see e.g.

  http://autobuild.buildroot.net/results/81a3bca5cbcf789c7ce1aa221a6a4154dd7c3917/

Instead of passing TARGET_ABI or TARGET_CFLAGS for libstdc++ in
--enable-cxx-flags parameter backport the patch that fixes propagation
of CXXFLAGS_FOR_TARGET to CXXFLAGS.

This issue is fixed in gcc-5.x

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>