fix undefined reference to bfd_link_plugin_object_p during link

fix undefined reference to bfd_link_plugin_object_p during link

When configured with the default options, GDB currently fails to link,
due to an undefined reference to bfd_link_plugin_object_p, coming from
elflink.c:

    #ifdef BFD_SUPPORTS_PLUGINS
           || (abfd->plugin_format == bfd_plugin_unknown
              && bfd_link_plugin_object_p (abfd))
    #endif

This is because BFD_SUPPORTS_PLUGINS is always defined. It is its value
that determines whether plugin support is enabled or not.

bfd/ChangeLog:

        * elflink.c: Check the value of BFD_SUPPORTS_PLUGINS rather
        than its existance.