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.