[X86] Change getHostCPUName fallback code to not select 'x86-64' for unknown CPUs...

[X86] Change getHostCPUName fallback code to not select 'x86-64' for unknown CPUs in family 6 that has 64-bit support but not any newer SSE features. Use 'core2' instead

We know that's the earliest CPU with 64-bit support. x86-64 has taken on a role of representing a more modern 64-bit CPU so we probably shouldn't be using that when we can't identify things.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317229 91177308-0d34-0410-b5e6-96231b3b80d8