Use the cpuid 64 bit flag to pick the default CPU name for an unknown model.

Use the cpuid 64 bit flag to pick the default CPU name for an unknown model.

For the Family 6 switch in sys::getHostCPUName, an unrecognized model was
reported as "i686".  That's a really bad default since it means that new
CPUs will be treated as if they can only use 32-bit code.  This just looks
at the cpuid extended feature flag for 64 bit support, and if that is set,
it uses a default x86-64 cpu.  Similar logic is already used for the Family
15 code.  <rdar://problem/11314502>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156486 91177308-0d34-0410-b5e6-96231b3b80d8