[gold patch 2/3] Support for incremental linking: write incremental got_plt info

[Cary Coutant]

> This is interesting, because it means that if you change the values qof
> the GOT_TYPE_ enums in the gold source code, existing incremental input
> files will fail in surprising ways.  That is, it makes the specific enum
> values part of the incremental linking ABI.  We won't remember to change
> the incremental link version every time we change an enum value.  On the
> other hand, it's fairly unlikely that we will ever change an enum
> value.  Any suggestion on how to make this more foolproof?q
>
>
> This is OK.

I've updated the copyright years on these files as well, added
comments to the Got_type enums, and committed. I'll add unit tests for
the Got_type enums separately.

-cary