XFAIL this on freebsd to bring the bot back.

Joerg Sonnenberger tells me one can open a directory in freebsd. I will try
to centralize our calls to open so that we can handle O_BINARY in one place,
and will then handle this there too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186317 91177308-0d34-0410-b5e6-96231b3b80d8