Readding FreeBSD support to lit.cfg. Patch from Pawel Worach.

Pawel has been using this patch on his buildbots for a while. This should
allow the testsuite to run on FreeBSD with libcxxrt.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215787 91177308-0d34-0410-b5e6-96231b3b80d8