[Driver][Mips] If CPU name is not provided to the driver explicitly use
multilibs from the FSFS toolchain corresponding to the mips32r2/mips64r2 CPUs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213175 91177308-0d34-0410-b5e6-96231b3b80d8