Make stage2 start.zig work on Windows

lib/std/start.zig

@@ -28,6 +28,8 @@ comptime {
                 if (@typeInfo(@TypeOf(root.main)).Fn.calling_convention != .C) {
                     @export(main2, .{ .name = "main" });
                 }
+            } else if (builtin.stage2_os == .windows) {
+                @export(wWinMainCRTStartup2, .{ .name = "wWinMainCRTStartup" });
             } else {
                 if (!@hasDecl(root, "_start")) {
                     @export(_start2, .{ .name = "_start" });
@@ -96,6 +98,11 @@ fn callMain2() noreturn {
     exit2(0);
 }
 
+fn wWinMainCRTStartup2() callconv(.C) noreturn {
+    root.main();
+    exit2(0);
+}
+
 fn exit2(code: usize) noreturn {
     switch (native_os) {
         .linux => switch (builtin.stage2_arch) {
@@ -148,11 +155,16 @@ fn exit2(code: usize) noreturn {
             },
             else => @compileError("TODO"),
         },
+        .windows => {
+            ExitProcess(@truncate(u32, code));
+        },
         else => @compileError("TODO"),
     }
     unreachable;
 }
 
+extern "kernel32" fn ExitProcess(exit_code: u32) callconv(.C) noreturn;
+
 ////////////////////////////////////////////////////////////////////////////////
 
 fn _DllMainCRTStartup(