[Mips] Support FSF Mips toolchain directories tree in the Clang driver.

The patch reviewed by Rafael Espindola.
http://llvm-reviews.chandlerc.com/D1843

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192343 91177308-0d34-0410-b5e6-96231b3b80d8