Fix setup_libstdcxx() in scripts/loader.py

Fix the following breakage caused by commit c98b928 ("Explicitly use python
version number") when starting GDB:

  Traceback (most recent call last):
    File "/home/penberg/osv/scripts/loader.py", line 833, in <module>
      setup_libstdcxx()
    File "/home/penberg/osv/scripts/loader.py", line 496, in setup_libstdcxx

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>