When an OutputSection is larger than the branch range for a Target we need to place thunks such that they are always in range of their caller, and sufficiently spaced to maximise the number of callers that can use the thunk. For ARMv7a we use the simple heuristic of placing the ThunkSection at intervals corresponding to the Thumb2 branch range. If the OutputSection is small we put the thunks at the end of the executable sections.
There are many alternative strategies for Thunk placement to maximize reuse. I've gone for a simple implementation that places the majority of Thunks slightly less than the most common branch range into the OutputSection. I think that this will work well in the most common case, a single .text OutputSection.