Replace all references to "Win32" with "Windows".

For historical reasons, "Win32" has been used in Rust codebase to mean "Windows OS in general".
This is confusing, especially now, that Rust supports Win64 builds.

[breaking-change]
Replace #[cfg(target_os = "win32")] with #[cfg(target_os = "windows")]
Replace "ignore-win32" in tests with "ignore-windows"
auto merge of #16435 : vadimcn/rust/windows, r=brson

Using "win32" to mean "Windows" is confusing, especially now, that Rust supports win64 builds.
Let's call spade a spade.