Pointer comparators and difference match the C standard.

Fixed two issues with pointer operations.
C11 6.5.6.9: Ptr subtraction have type ptrdiff_t.
C11 6.5.8.6, 6.5.9.3: Ptr comparisons have type int.
This was my mistake though, I should have made sure it matched 8cc/include/stddef.h which is currently "long", on my redhat machine its actually declared as "typedef long int ptrdiff_t".

To be honest, the C types with all the variations in declarations seem like legacy cruft to me.
long long, long, long int etc etc etc.