Make atomic_fetch_add() and atomic_fetch_sub() work for pointers with GCC 4.2.

According to the standard, atomic_fetch_*() has to behave identical to
regular arithmetic. This means that for pointer types, we have to apply
the stride when doing addition/subtraction.

The GCC documentation seems to imply this is done for __sync_*() as
well. Unfortunately, both tests and Googling seems to reveal this is not
really the case. Fix this by performing the multiplication with the
stride manually.