Allow pretty-print of static members

Python pretty-printers haven't applied to static members for quite
some time.  I tracked this down to the call to cp_print_value_fields
in cp_print_static_field -- it doesn't let pretty-printers have a
chance to print the value.  This patch fixes the problem.

The way that static members are handled is very weird to me.  I tend
to think this should be done more globally, like in value_print.
However, I haven't made any big change.

Reviewed-by: Keith Seitz <keiths@redhat.com>
Tested-by: Keith Seitz <keiths@redhat.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30057