Try to fix windows build more after r198136.

The command that cmd.exe is complaining about is:
cmd.exe /c cd /D C:\bb-win7\cmake-clang-i686-mingw32\build\tools\lto && cmake -E echo EXPORTS > symbol.def && type C:/bb-win7/cmake-clang-i686-mingw32/llvm-project/llvm/tools/lto/lto.exports >> symbol.def

Maybe quoting the filename helps.