backport plugin fixes to 2.21

[Richard Sandiford]

Alan Modra <amodra@gmail.com> writes:
> I've backported my plugin fixes to the branch, along with some from
> HJ and Dave.  I also put Mark Wielaard's PR 10549 fix on the branch,
> with the exception of the ARM patch.  I left ARM out since the branch
> lacks ifunc support for ARM, and I didn't want to make the call that
> ifunc was safe/desirable for ARM on the branch.  That's not to say
> that I think it would be unsafe, just that I don't know.

Yeah, the ifunc patches were quite invasive, so I think it'd be
better to leave them out.

Richard