[libcxxabi][CMake] Support in-tree libunwind when building as part of runtimes

When building as part of runtimes, there is no predefined order in
which the runtimes are loaded, so the targets from other projects
might not be available. We need to rely on HAVE_<name> variables
instead in that case.

Differential Revision: https://reviews.llvm.org/D29574

llvm-svn: 294552