Improve support for private platforms (#11220)

src/SDL_assert.c

@@ -245,7 +245,9 @@ static SDL_AssertState SDLCALL SDL_PromptAssertion(const SDL_AssertData *data, v
             state = (SDL_AssertState)selected;
         }
     } else {
-#ifdef SDL_PLATFORM_EMSCRIPTEN
+#ifdef SDL_PLATFORM_PRIVATE_ASSERT
+        SDL_PRIVATE_PROMPTASSERTION();
+#elif defined(SDL_PLATFORM_EMSCRIPTEN)
         // This is nasty, but we can't block on a custom UI.
         for (;;) {
             bool okay = true;