Fix statistics for x86 Windows builds

Fix statistics for x86 Windows builds

Using atomic_int_fast64_t variables with atomic functions on x86 does
not cause Visual Studio to report build errors, but such operations
yield useless results.  Since the isc_stat_t type is unconditionally
typedef'd to atomic_int_fast64_t, any code performing atomic operations
on isc_stat_t variables is broken in x86 Windows builds.  Fix by using
the atomic_int_fast32_t type for isc_stat_t in x86 Windows builds.