loader: fix undefined behavior in rom_order_compare()

According to ISO C99 / N1256 (referenced in HACKING):

> 6.5.8 Relational operators
>
> 4 For the purposes of these operators, a pointer to an object that is
>   not an element of an array behaves the same as a pointer to the first
>   element of an array of length one with the type of the object as its
>   element type.
>
> 5 When two pointers are compared, the result depends on the relative
>   locations in the address space of the objects pointed to. If two
>   pointers to object or incomplete types both point to the same object,
>   or both point one past the last element of the same array object, they
>   compare equal. If the objects pointed to are members of the same
>   aggregate object, pointers to structure members declared later compare
>   greater than pointers to members declared earlier in the structure,
>   and pointers to array elements with larger subscript values compare
>   greater than pointers to elements of the same array with lower
>   subscript values. All pointers to members of the same union object
>   compare equal. If the expression /P/ points to an element of an array
>   object and the expression /Q/ points to the last element of the same
>   array object, the pointer expression /Q+1/ compares greater than /P/.
>   In all other cases, the behavior is undefined.

Our AddressSpace objects are allocated generally individually, and kept in
the "address_spaces" linked list, so we mustn't compare their addresses
with relops.

Convert the pointers subjected to the relop in rom_order_compare() to
"uintptr_t":

> 7.18.1.4 Integer types capable of holding object pointers
>
> 1 [...]
>
>   The following type designates an unsigned integer type with the
>   property that any valid pointer to void can be converted to this type,
>   then converted back to pointer to void, and the result will compare
>   equal to the original pointer:
>
>   /uintptr_t/
>
>   These types are optional.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Alistair Francis <alistair.francis@xilinx.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-devel@nongnu.org
Fixes: 3e76099aacb4dae0d37ebf95305369e03d1491e6
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
