gdb/testsuite: make gdb_supported_languages a caching proc

Rewrite gdb_supported_languages as a caching proc that actually
queries GDB for the list of supported languages, rather than just
containing a hard-coded list of languages.

There's only one test that uses this proc right now,
gdb.python/py-function.exp, and that still passes after this change,
with no changes in the test names.