[X86] Promote athlon, athlon-xp, k8, and k8-sse3 to types instead of subtypes...

[X86] Promote athlon, athlon-xp, k8, and k8-sse3 to types instead of subtypes in getHostCPUName. NFCI

This removes the athlon type and simplifies the string decoding. We only really need these type/subtype breaks where we need to match libgcc/compiler-rt and these CPUs aren't part of that.

I'm looking into moving some of this information to a .def file to share with clang's __builtin_cpu_is handling. And while these CPUs aren't part of that the less lines I have to deal with in the .def file the better.

llvm-svn: 317354