X86: Remove TargetMachine CPU auto-detection.

This logic is properly in the realm of whatever is creating the
TargetMachine. This makes plain 'llc foo.ll' consistent across
heterogenous machines.
llc: Add support for -mcpu=native.

When -mcpu=native is passed, autodetect the host CPU and pass that
as the CPU name to the TargetMachine factory method.