[lldb] Group ABI plugins

Summary:
There's a fair amount of code duplication between the different ABI plugins for
the same architecture (e.g. ABIMacOSX_arm & ABISysV_arm). Deduplicating this
code is not very easy at the moment because there is no good place where to put
the common code.

Instead of creating more plugins, this patch reduces their number by grouping
similar plugins into a single folder/plugin. This makes it easy to extract
common code to a (e.g.) base class, which can then live in the same folder.

The grouping is done based on the underlying llvm target for that architecture,
because the plugins already require this for their operation.

Reviewers: JDevlieghere, jasonmolenda, jfb

Subscribers: sdardis, nemanjai, mgorny, kristof.beyls, fedor.sergeev, kbarton, jrtc27, atanasyan, jsji, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74138