[libcxxabi] [test] Depend on unwind only if available

When building libcxxabi via LLVM_ENABLE_RUNTIMES=libcxxabi the CMake
invocation fails because of missing "unwind" target.  However,
if the extraneous dependency is removed, the library builds just fine
against installed libunwind and tests work fine.  To fix this,
add the dependency only if the target actually exists.

Differential Revision: https://reviews.llvm.org/D119538