readelf: Use dwarf_form_name for printing (unknown) forms.

readelf: Use dwarf_form_name for printing (unknown) forms.

Use dwarf_form_name consistently to print forms. attr_callback was
using its own string conversion, print_debug_macro_section was using
dwarf_form_string instead of dwarf_form_name.

The difference between dwarf_form_string and dwarf_form_name is that
the first returns NULL if the form is unknown and the second returns
an hexadecimal string representation in that case.

Signed-off-by: Mark Wielaard <mark@klomp.org>