Move StopInfoOverride callback to the new architecture plugin

This creates a new Architecture plugin and moves the stop info override
callback to this place. The motivation for this is to remove complex
dependencies from the ArchSpec class because it is used in a lot of
places that (should) know nothing about Process instances and StopInfo
objects.

I also add a test for the functionality covered by the override
callback.

Differential Revision: https://reviews.llvm.org/D31172

llvm-svn: 316609