[mips] Add support MIPS r6 Debian triples

Debian uses different triples for MIPS r6 and paths. Here we use SubArch
to determine whether it is r6, if we found `r6' in CPU section of triple.

These new triples include:
  mipsisa32r6-linux-gnu
  mipsisa32r6el-linux-gnu
  mipsisa64r6-linux-gnuabi64
  mipsisa64r6el-linux-gnuabi64
  mipsisa64r6-linux-gnuabin32
  mipsisa64r6el-linux-gnuabin32

Patch by YunQiang Su.

Differential revision: https://reviews.llvm.org/D50857

llvm-svn: 343185