Plugin tests in ld testsuite

[Stephen Casner]

On Sun, 21 Jun 2020, Alan Modra wrote:

> I'm aiming to fix that PR by firstly making all files in ld/ that
> currently test ENABLE_PLUGINS instead test BFD_SUPPORTS_PLUGINS.  That
> will allow just one configure file to control the default for whether
> plugins are enabled.  I think it would be reasonable for some targets
> like pdp11 and alpha-vms to have plugins disabled by default, with
> users getting a warning if they override the default with
> --enable-plugins.

Thank you for taking care of my problem at the same time!

                                                        -- Steve