x86: Use simple nop codes for <= sse (rather than <= mmx)

The "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.

Signed-off-by: Martin Storsjö <martin@martin.st>