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.
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