[dmd-internals] Refactoring of the Objective-C integration

[Walter Bright via dmd-internals]

On 3/4/2017 11:38 PM, Sebastian Wilzbach via dmd-internals wrote:
> FWIW the stubbing also creates pain on the tooling end as these files need to be
> manually excluded on all depending pipelines.

We already do this for libelf.d, etc. I don't recall it ever being an issue.

Target dependent object file generation does not use a runtime switch, either.

The stub file can be merged with the non-stub file, and use a compile time 
version conditional. No need for runtime dispatch.