Demoting i686 Windows targets to std-only

With the release of Rust 1.100.0, changes to 32-bit Windows targets include demoting the i686-pc-windows-msvc and i686-pc-windows-gnu targets without host tools. The development for these platforms is becoming obsolete due to the lack of modern 32-bit hardware and issues encountered in building toolchains. Consequently, users will need to cross-compile from a 64-bit host instead of using 32-bit Windows hosts, although the standard library will still be available for now.