Add a new subtarget hook for whether or not we'd like to enable
the atomic load linked expander pass to run for a particular
subtarget. This requires a check of the subtarget and so save
the TargetMachine rather than only TargetLoweringInfo and update
all callers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211314 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211315 91177308-0d34-0410-b5e6-96231b3b80d8