support coarse-grain and persisted fragments on ARM

Split from issue #1551

This covers adding support for coarse-grain and persisted fragments on ARM, which we will be punting on initially.

Start by reading issue #1551, from which this work was split, and any discussion connected to ARM support. Define the existing entry points for coarse-grain and persisted fragments before estimating the change; done means both fragment modes are supported on ARM and validated by the project's relevant tests.

Written by the indexing model from the issue text.