C++: Fix vtable with libstdc++

__cxxabiv1 is an internal namespace used by GNU's libstdc++.
When linking with C++ standard library (libstdc++),
__cxxabiv1::__class_type_info is already defined inside.
If user code contains virtual classes,
__cxxabiv1::__class_type_info is defined twice.

Signed-off-by: Benoit Leforestier <benoit.leforestier@gmail.com>