bug#53894: 27.2; Clipboard copy -> C-y -> M-y -> Same clipboard copy does not push to kill ring

[Po Lu]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> platforms) now, since Po Lu introduced the
> `gui-backend-selection-owner-p' function), we should.

Minor correction: I didn't introduce that function, I only made
`gui-selection-value' take advantage of it on platforms where it works.