Use simple nop codes for <= sse (rather than <= mmx).

The "CPU: CentaurHauls family 6 model 9 stepping 8" family of CPUs
(flags: fpu vme de pse tsc msr cx8 sep mtrr pge mov pat mmx fxsr sse
up rng rng_en ace ace_en) SIGILLs on long nop codes.

Change-Id: I7e7c52a2191006df30a9aadbc40d481a1db89106