Use --enable-obsolete in build-many-glibcs.py for nios2-linux-gnu

Until GCC removes Nios II support (at which point we should do so as
well), this is now needed for GCC 14 / mainline to build for
nios2-linux-gnu target.

Tested with build-many-glibcs.py (GCC mainline) for nios2-linux-gnu.