[libcxx] Add a dependency on unwind in cxx_static, if LIBCXXABI_USE_LLVM_UNWINDER is set

Even if building cxx_static in itself doesn't actually link in the
requested unwinder, add a synthetic dependency so that building
cxx_static makes sure that the unwinder that was requested to be used
also gets built.

This makes sure that tests (when run with just a plain "ninja check-cxx")
actually use the newly built unwinder, as intended.

Differential Revision: https://reviews.llvm.org/D113467