[X86] Simplify the pentium4 code in getHostCPUName to be based on feature...

[X86] Simplify the pentium4 code in getHostCPUName to be based on feature flags. Don't use 'x86-64' ever.

'x86-64' has started to reflect a sort of generic tuning flag for more modern 64-bit CPUs. We probably shouldn't be using it as the name of an unidentifiable pentium4. So use nocona for all 64-bit pentium4s instead.

llvm-svn: 317230