Makefile is too architecture specific while it should not

[Narann]

@Nebuleon tried to compile color combiner refactoring and get this error:

Makefile:118: *** CPU type "mips" not supported. Please file bug report at 'http://code.google.com/p/mupen64plus/issues'. Stop.
make[1]: Leaving directory `/storage/programming/mupen64plus/mupen64plus-video-rice/projects/unix'
because I build with parallel make, other projects built fine, but not this one
Your project is not architecture-specific; it's disingenuous to have the checks. I undid some of them in commits like mupen64plus/mupen64plus-input-sdl@630e4c7

As Rice does not rely on arch specific opcode, we should remove the check from the Makefile

[Narann]

Thanks Nebulon. I'm not a compilation expert so I would wait for @richard42 comment on this before accept the PR. :)