[X86] Add FeatureCMOV to athlon and athlon-tbird cpus.

Summary: This matches gcc and one cpuid dump I found online. Given that these are considered 7th generation x86 CPU it seems likely they support cmov since cmov was added by Intel in their 6th generation.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51264

llvm-svn: 340706