Use libc++ on FreeBSD as on Darwin

This could use some refinement still, but the previous behaviour of adding
-stdlib=libstc++ on FreeBSD w/ Clang is the least likely case to work.

llvm.org/pr17910


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@200646 91177308-0d34-0410-b5e6-96231b3b80d8