Use larger type than long for pointers for win64 model

[Martin Packman]

Various code in meliae assumes that the long type is large enough to hold a pointer value, this isn't true in the LLP64 model used by win64. This is acknowledged by the code:

    /* TODO: This is casting a pointer into an unsigned long, which we assume
     * is 'long enough'. We probably should really be using uintptr_t or
     * something like that.
     */

The one pitfall with just correcting to a platform appropriate type is that text dumps generated on 64-bit machine may not be loadable on a 32-bit machine. Depending on whether the extra memory usage is problematic or not, always using an 8 byte value in the loader code would get around that problem,