Add assert for stub platform on windows host.

The stub platform currently can not be compiled on windows and instead shows
cryptic gn errors.

To avoid this confusion, assert when trying to configure stub platform on a
windows host.

starboard/stub/platform_configuration/configuration.gni

@@ -30,5 +30,6 @@ no_pedantic_warnings_config_path =
     "//starboard/stub/platform_configuration:no_pedantic_warnings"
 
 is_clang_16 = true
+assert(!is_host_win, "Stub build is not (yet) supported on windows host")
 
 v8_enable_webassembly = true