[PATCH] [RISCV] Extend getTargetDefines for RISCVTargetInfo

Summary:
This patch extend getTargetDefines and implement handleTargetFeatures
and hasFeature. and define corresponding marco for those features.

Reviewers: asb, apazos, eli.friedman

Differential Revision: https://reviews.llvm.org/D44727

Patch by Kito Cheng.

llvm-svn: 329278