Update LTO plugin interface

[Ian Lance Taylor]

"H.J. Lu" <hjl.tools@gmail.com> writes:

> I am not saying we should go with 2 stage linking. Just for clarification.
> In 2 stage linking, the first stage only does symbol resolution. The complete
> linking starts at the second stage.  So we don't do really complete relink
> in 2 stage linking.

Understood.  But you still read and process all the symbols twice.  It
can be done if it must be done, but it would be significantly better to
do something else if possible.

Ian