On LLP64 systems (like 64-bit Windows) long is 32 bits while pointers
are 64 bits, which results in a warning similar to the following:

warning C4244: 'return': conversion from '__int64' to 'long', possible loss of data

for HAVE_STACK_GROWS_UPWARDS.  Fix this by using the ptrdiff_t type
introduced by C99 for this purpose.
Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>