[PATCH v3] mscoree: Implement VTable fixup for x86_64 architecture.

[Vincent Povirk]

> I would suggest to move those changes and some of the changes below into a separate patch.
> They can easily be reviewed separately and using PtrToUint() instead of pointer types wasn't
> even a good idea on 32-bit. ;)

The original idea was that we'd have a separate loop for vtables with
the COR_VTABLE_64BIT flag, and on 32-bit we'd support both flags.
Making the flag checked arch-specific means we can use a pointer type,
but it also means we can't handle a 64-bit vtable fixup in 32-bit
images (which native may not even allow, I haven't checked).

We really should've had some sort of "unsupported flags" check in
there, but I didn't want to bring up pre-existing issues.

The reason we didn't allocate a proper stack frame on x86 is that I
didn't realize it was an option.