Fix SDL2main on PSP (#5076)

- Fix SDL2main on PSP
  SDL2main was not working for PSP, because it wasn't being activated and
  it wasn't unsetting the main. Besides that a debug screen being started
  was causing issues with joystick input and the sceKernelExitGame calli
  is no longer needed with the current PSPDEV SDK.
- Clean up imports in PSP main
- Set PSP GPU and user modes in main
- Fix exit callback in PSP main

include/SDL_main.h

@@ -83,6 +83,15 @@
 */
 #define SDL_MAIN_NEEDED
 
+#elif defined(__PSP__)
+/* On PSP SDL provides a main function that sets the module info,
+   activates the GPU and starts the thread required to be able to exit
+   the software.
+
+   If you provide this yourself, you may define SDL_MAIN_HANDLED
+ */
+#define SDL_MAIN_AVAILABLE
+
 #endif
 #endif /* SDL_MAIN_HANDLED */