Statistics are broken in 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.

This was introduced in BIND 9.13.3, by commit e9e55cbd, which is part of !657 (merged). Every x86 Windows build produced since then has broken statistics.