Enable MCJIT on FreeBSD

Testing shows it works for at least trivial cases, while the
USE_STANDARD_JIT case does not even work for those.  Thus, don't define
USE_STANDARD_JIT on FreeBSD.

I've left the #if block choosing the appropriate #include in case it's
useful for testing.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189611 91177308-0d34-0410-b5e6-96231b3b80d8