Add MCSymbolCOFF

[Rafael Espíndola]

>> Why is the common symbol handling that it removes a nop?
> I moved all the logic for getting the encoded flags in to a method on the MCSymbolMachO class.  The common stuff was just part of that.  This was just to prevent leaking any of the flags from the MCSymbolMachO class, but i can remove that part if you prefer I leave it as it was.  I can make the required flags/mask accessible from MCSymbolMachO if needed.

Now I see it. It is fine in patch 6 with the other changes.

Cheers,
Rafael