Enable TTI for host TargetMachine in JitRunner

This commit improves JitRunner so that it creates a target machine
for the current CPU host which is used to properly initialize LLVM's
TargetTransformInfo for such a target. This will enable optimizations
such as vectorization in LLVM when using JitRunner. Please, note that,
as part of this work, JITTargetMachineBuilder::detectHost() has been
extended to include the host CPU name and sub-target features as part of
the host CPU detection (https://reviews.llvm.org/D65760).

Closes tensorflow/mlir#71

PiperOrigin-RevId: 262452525