compiling on mingw is now supported (#1542)

* compiles on mingw-w64
* fixed error in os_file_overwrite on windows
* fixed windows hello_world example

example/hello_world/hello_windows.zig

@@ -1,5 +1,7 @@
 use @import("std").os.windows;
 
+extern "user32" stdcallcc fn MessageBoxA(hWnd: ?HANDLE, lpText: ?LPCTSTR, lpCaption: ?LPCTSTR, uType: UINT) c_int;
+
 export fn WinMain(hInstance: HINSTANCE, hPrevInstance: HINSTANCE, lpCmdLine: PWSTR, nCmdShow: INT) INT {
     _ = MessageBoxA(null, c"hello", c"title", 0);
     return 0;