debugedit: Refactor reading/writing of relocated values.

debugedit: Refactor reading/writing of relocated values.

This refactors the reading and writing of relocated values into seperate
helper functions (setup_relbuf and update_rela_data). It will be easier
to reuse this code in case we want to read/write relocated values in other
sections than DEBUG_INFO. The only functional change is that we explicitly
track whether the relocation data is updated, and only explicitly update
and write out the relocation data if so. In the case there were no strp
or stmt updates, there will also not be any relocation updates, even if
there is relocation data available.

All new debugedit testcases pass before and after this refactoring.