atomic_windows_x86.hpp disables warning 4035 more widely than needed

atomic_windows_x86.hpp disables warning 4035 (missing return statement) over most (though not all) of the file. That warning only needs to be disabled over the !defined(AMD64) part of the file though, to deal with the code that uses __asm statements. The scope of the warning suppression should be narrowed accordingly.

The suppression should probably also be scoped by using the warning push/pop mechanism, rather than resetting to the default, which might not have been the original state.