Part of preparation for upstreaming a Win32 application runtime (see discussion #2563). This is one of three small build-related fixes that unblock the Windows GNU-ABI library build. When targeting Windows with GNU ABI, the existing `DllMain` declaration falls through to `void` (a type), which Zig stdlib's `start.zig` then attempts to call as a function via `root.DllMain(...)` - producing the compile error "type 'type' not a function". Restructure the conditional so that: - non-Windows builds keep `DllMain = void` - Windows + MSVC keeps the existing CRT-init handler (unchanged) - Windows + non-MSVC gets a no-op `BOOL` handler This unblocks `zig build -Dtarget=native-native-gnu -Dapp-runtime=none` on Windows.