[Driver][Mips] Remove "fp64" directories from the mips-mti-linux-gnu toolchain
directories description. Released version of this toolchain has not separate
libraries for -mfp64 command line option.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213937 91177308-0d34-0410-b5e6-96231b3b80d8