[J-core] Debug jump log?

[Rob Landley]

What would it take to add a debug feature to j-core which recorded the
address each jump came from into a ring buffer when we set a status
flag? I.E "start debugging mode", then run until breakpoint, then dump
the buffer to have all the "come from" addresses so we can reconstruct
the path that got us here back into the runtime history?

Even if it was writing the data straight into DRAM (thus slowing
execution down a lot) it would be a really useful feature for things
like profiling the interrupt path. (Doing that would need a register to
set the size/location of the buffer...)

Rob