demmt/g80: compute offset separately from start id
demmt/g80: compute offset separately from start id

This allows programs to have the correct start id displayed, which
matters for relocated jump labels.

envytools/envytools,grate-driver/envytools,grate-driver/envytools,MoochMcGee/envytools,hakzsam/envytools,MoochMcGee/envytools,kfractal/envytools,grate-driver/envytools,karolherbst/envytools,hakzsam/envytools,MoochMcGee/envytools,grate-driver/envytools,envytools/envytools,envytools/envytools,pierremoreau/envytools,karolherbst/envytools,pierremoreau/envytools,pierremoreau/envytools,envytools/envytools,karolherbst/envytools,kfractal/envytools,kfractal/envytools,kfractal/envytools,MoochMcGee/envytools,hakzsam/envytools,karolherbst/envytools,grate-driver/envytools,pierremoreau/envytools,karolherbst/envytools,MoochMcGee/envytools,hakzsam/envytools,kfractal/envytools,hakzsam/envytools,envytools/envytools,pierremoreau/envytools