[Mips] Do not detect a used MIPS toolchain. Build a path suffix for FSF

toolchain first and check the path existence. If the path does not
exist build and check a path suffix for Code Sourcery toolchain.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195751 91177308-0d34-0410-b5e6-96231b3b80d8