[driver][mips] Adjust target triple's environment accordingly to provided ABI name

For MIPS we need to adjust not only architecture name accordingly to ABI
provided by the `-mabi` command line option, but also modify triple's
environment. For example, for `mips-linux-gnu` triple and `-mabi=n32`
option a correct final triple is `mips64-linux-gnuabin32`.

llvm-svn: 344603