[gdb/python] Note that python 3.6 assumes long long support

Starting with python 3.6, support for platforms without long long support
has been removed [1].

HAVE_LONG_LONG and PY_LONG_LONG are still defined, but only for compatibility,
so stop relying on them.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

[1] https://github.com/python/cpython/issues/72148