[X86] Add 'mmx' to all CPUs that have a version of 'sse' and weren't already enabling '3dnow'

All SSE capable CPUs have MMX. 3dnow implicitly enables MMX.

We have code that detects if sse is enabled and implicitly enables
MMX unless -mno-mmx is passed. So in most cases we were already
enabling MMX if march passed a CPU that supported SSE.

The exception to this is if you pass -march for a cpu supports SSE
and also pass -mno-sse. We should still enable MMX since its part
of the CPU capability.