[cmake] Build lldb-server on Darwin, set dep on FreeBSD.

Summary:
This switches the decision as to whether or not to lldb-server should
be built to check the same flag that was added that controls whether
or not it is added as a dependency to the 'lldb' target.

It also sets that flag on FreeBSD to maintain parity with the existing
build configuration / situation on FreeBSD.

Reviewers: labath, emaste, tfiala

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12925

llvm-svn: 247913