need to flush icache on code cache memory re-use on ARM

Today we flush just on hot-patching branches for linking and unlinking.
We also need to flush big regions if we ever re-use them for new fragments.

For cases like cache resets, we'll have to decide whether this is simpler
to do on freeing, or whether we need to maintain a data structure that tracks
whether we've ever had code in each unit.

Locate the ARM code-cache reuse paths and the existing instruction-cache flushes used for hot-patching branches. Compare handling during cache reset and region freeing, then determine how reused regions can be identified and flushed; done means newly allocated fragments never execute stale instructions after reuse.

Written by the indexing model from the issue text.