Use lib/LTO directly in the gold plugin.

The tools/lto API is not the best choice for implementing a gold plugin. Among
other issues:

* It is an stable ABI. Old errors stay and we have to be really careful
  before adding new features.
* It has to support two fairly different linkers: gold and ld64.
* We end up with a plugin that depends on a shared lib, something quiet
  unusual in LLVM land.
* It hides LLVM. For some features in the gold plugin it would be really
  nice to be able to just get a Module or a GlobalValue.

This change is intended to be a very direct translation from the C API. It
will just enable other fixes and cleanups.

Tested with a LTO bootstrap on linux.