system: Use AVR timer to generate millisecond count

Used in get_ticks().
Change #ifdefs to check for __AVR__ instead of __i386__

This way both __i386__ and __x86_64__ are supported by the non-AVR code.

Also removed SDL_Surface entirely from system.h for AVR.
Also added a compiler error if the system is neither x86 nor AVR.  This
should catch cases where the implementation is something else (e.g. ARM or
PowerPC) that might require another handling case.