[libcxx] Force-cache LIBCXX_CXX_ABI_LIBRARY_PATH

Summary:
The `LIBCXX_CXX_ABI_LIBRARY_PATH` CMake variable is cached once in
libcxx/cmake/Modules/HandleLibCXXABI.cmake in the `setup_abi_lib` macro,
and then cached again in libcxx/test/CMakeLists.txt. There, if it is
not set to a value, it is by default set to `LIBCXX_LIBRARY_DIR`.

However, this new value is not actually cached, because the old (empty)
value has been already cached. Use the `FORCE` CMake flag so that it
is saved to the cache.

This should not break anything, because the code changed here previously
had no effect, when it should have.

Reviewers: jroelofs, bcraig, ldionne, EricWF, mclow.lists, vvereschaka, eastig

Reviewed By: vvereschaka

Subscribers: mgorny, christof, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D69169