[X86] Add FeatureCMPXCHG8B and FeatureSlowUAMem16 to 'lakemont' in X86.td

We already had CMPXCH8B feature on this CPU for the frontend so
this doesn't have much effect.

The FeatureSlowUAMem16 only matters if someone compiles with
-march=lakemont -msse which doesn't make sense, but is consistent
with all our pre-sse4.2 CPUs. Maybe the feature flag should be
FeatureFastUAMem16 and set on the newer CPUs instead.