sdl: fix freeze on startup on PP targets

Using the coprocessor was a good idea in theory, but didn't actually work.
This moves all SDL threads to the main core, which isn't ideal, but at
least works. Additionally, this also adds some good-practice stuff such as
setting the default sample rate on exit.

Change-Id: If1636b33d439000ec7c4e50f46e8d002708d3121

apps/plugins/sdl/src/audio/rockbox/SDL_rockboxaudio.c

@@ -222,6 +222,7 @@ static void ROCKBOXAUD_CloseAudio(_THIS)
             SDL_FreeAudioMem(this->hidden->rb_buf[i]);
     }
     rb->pcm_play_stop();
+    rb->pcm_set_frequency(HW_SAMPR_DEFAULT);
 }
 
 static int ROCKBOXAUD_OpenAudio(_THIS, SDL_AudioSpec *spec)