Plugin interfaces to do Pettis Hansen style code layout in the gold linker.

[Cary Coutant]

> However, I don't see why you are using an index here.  You have an
> Object* pointer.  Why not just use that as the handle?

Sri's first patch did use the Object* as the handle, but I suggested
using an index like the original plugin APIs do. This makes it more
consistent with the existing interfaces, and also makes it easier to
sanity check the value passed in from a plugin library.

-cary