Add assert for stub platform on windows host. (youtube#1906)

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