VM: Fix Windows VM build following type renaming

Making cell be a typedef of uintptr_t broke a spot assigning
cell* was unsigned long*. Added an explicit cast.
VM: Fix Windows 64-bit build on Visual Studio 2012

The UNW_FLAG_EHANDLER #define is now visible in VS2012, causing
a conflict with the definition in os-windows-x86.64.cpp. Added
a #ifndef to only include the definition if it hasn't already
been defined.