Some linker libraries are only needed for shared libc++, some only
for static libc++, combining these together in LIBCXX_LIBRARIES and
LIBCXX_INTERFACE_LIBRARIES can introduce unnecessary dependencies.
This changes splits those up into LIBCXX_SHARED_LIBRARIES and
LIBCXX_STATIC_LIBRARIES matching what libc++abi already does. This
also allowed eliminating LIBCXX_INTERFACE_LIBRARIES which is no longer
needed.