package/opencv3: fix CMAKE_CXX_FLAGS

The commit 4904c4ce "package/opencv3: use BR2_TOOLCHAIN_HAS_LIBATOMIC"
overrides CMAKE_CXX_FLAGS with the single -latomic, losing all ABI
CFLAGS that are passed there by default. This breaks build on xtensa
where ABI CFLAGS contain important code generation options.

Append $(TARGET_CXXFLAGS) to CMAKE_CXX_FLAGS along with -latomic.

Fixes:
  http://autobuild.buildroot.net/results/7f1c96abd8fbb5b358a07100ab623316e9bb9dcd
  http://autobuild.buildroot.net/results/e0c93d0f6d1da0d62d4dbba211a275bfe75e9645
  http://autobuild.buildroot.net/results/53e7e4b4b6a7b48b8012799d7507f7594dbf01b2



Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>