Bring back the dylib kind of the link attribute

Hello,


`dylib` [seems][1] to be no longer an option for the `kind` key of the `link` attribute.

UPDATE: It should be the other way around: It [seems][1] `dylib` has been lost as a possible variant of the `kind` key of the `link` attribute. See the comment below.

Regards,
Ivan

[1]: https://github.com/rust-lang/rust/blob/8f8753878644b0bfb38d24781edb9ef2028730f2/src/librustc/metadata/creader.rs#L237