bpo-34537: Fix test_gdb:test_strings with LC_ALL=C (GH-9483)

We cannot simply call locale.getpreferredencoding() here,
as GDB might have been linked against a different version
of Python with a different encoding and coercion policy
with respect to PEP 538 and PEP 540.

Thanks to Victor Stinner for a hint on how to fix this.