This flag should mangle as such:

ld: error: undefined symbol: v8::platform::NewDefaultPlatform(int,
v8::platform::IdleTaskSupport, v8::platform::InProcessStackDumping,
std::unique_ptr<v8::TracingController,
std::default_delete<v8::TracingController> >)

Instead of: (note the std::__Cr:: instead of std::)

000000000000f5e0 T v8::platform::NewDefaultPlatform(int,
v8::platform::IdleTaskSupport, v8::platform::InProcessStackDumping,
std::__Cr::unique_ptr<v8::TracingController,
std::__Cr::default_delete<v8::TracingController> >)