Get testsuite running on FreeBSD.

Add initial support for using LIT to run the tests on FreeBSD. 
More work may need to be done to add support for FreeBSD but this is a
good initial step.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215742 91177308-0d34-0410-b5e6-96231b3b80d8