[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

We could add target-specific functions to map the enum into a symbolic
value and back. Either way we end up with something that is baked into
the ABI, and I don't think that symbolic values would be so much more
stable than the enums that we have now to make it worth it. At least
the enums are explicitly assigned rather than auto-incremented. Would
a comment at the enum definition be sufficient?

-cary