[driver][mips] Support MIPS R6 target triples

This change adds support for the following MIPS target triples:
  mipsisa32r6-linux-gnu
  mipsisa32r6el-linux-gnu
  mipsisa64r6-linux-gnuabi64
  mipsisa64r6el-linux-gnuabi64
  mipsisa64r6-linux-gnuabin32
  mipsisa64r6el-linux-gnuabin32

Patch by Yun Qiang Su.

Differential revision: https://reviews.llvm.org/D50850

llvm-svn: 344608