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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322100 91177308-0d34-0410-b5e6-96231b3b80d8