[CMake][libcxx] Do not rely on the existence of c++abi or unwind targets

There is no guaranteed order in which CMake files for individual
runtimes are invoked and therefore we cannot rely on existence of
targets defined in other runtimes. Use the new HAVE_<name> options
instead in those cases.

Differential Revision: https://reviews.llvm.org/D28391

llvm-svn: 291632