[lli] Make lli support -mcpu=native for CPU autodetection

llc, opt, and clang can all autodetect the CPU and supported features. lli cannot as far as I could tell.

This patch uses the getCPUStr() and introduces a new getCPUFeatureList() and uses those in lli in place of MCPU and MAttrs.

Ideally, we would merge getCPUFeatureList and getCPUFeatureStr, but opt and llc need a string and lli wanted a list. Maybe we should just return the SubtargetFeature object and let the caller decide what it needs?

Differential Revision: https://reviews.llvm.org/D41833

llvm-svn: 322100