Fix global ::delete on virtual object

[DutChen18]

Using ::delete would fail because ItaniumCXXABI::emitVirtualObjectDelete used the wrong offset (-2) into the vtable to find the complete-object pointer.

Example (triggers address sanitizer):

struct Base {
        virtual ~Base() {}
};

struct Derived : Base {};

int main() {
        Derived* derived = ::new Derived();
        ::delete derived;
}

[yuri91]

I was incorrect in my explanation on discord: for dynamic_cast<void*> we generate a "virtualcast" for both asmjs and genericjs. It's just different the way we compute the offset (for genericjs it's always 0, for asmjs it is stored at index 0 in the vtable and needs to be loaded).

So here you are computing the offset correctly but then you are going to the regular logic for the pointer adjustment, while you still need the virtualcast. (it's possible that for asmjs/wasm it ends up giving you the exact same address, but for consistency let's use virtualcast for both cases)

[DutChen18]

How about this then?

[yuri91]

Looks good to me. I will wait the CI before merging.