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/thread/rockbox/SDL_systhread.c

@@ -58,7 +58,7 @@ int SDL_SYS_CreateThread(SDL_Thread *thread, void *args)
     thread->handle = rb->create_thread(rbsdl_runthread, stacks[threadnum], DEFAULT_STACK_SIZE,
                                        0, names[threadnum] /* collisions allowed? */
                                        IF_PRIO(, PRIORITY_USER_INTERFACE)
-                                       IF_COP(, COP));
+                                       IF_COP(, CPU));
 
     threadnum++;