gdb: make lookup_minimal_symbol objf and sfile parameters optional

Most calls to lookup_minimal_symbol don't pass a value for sfile and
objf.  Make these parameters optional (have a default value of
nullptr).  And since passing a value to `objf` is much more common than
passing a value to `sfile`, swap the order so `objf` comes first, to
avoid having to pass a nullptr value to `sfile` when wanting to pass a
value to `objf`.

Change-Id: I8e9cc6b942e593bec640f9dfd30f62786b0f5a27
Reviewed-by: Keith Seitz <keiths@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>