gas: consolidate . latching

gas: consolidate . latching

... by purging dot_{frag,value}. Right now these two and dot_symbol are
updated independently, which can't be quite right. Centralize .-related
information in dot_symbol, updating it also where previously
dot_{frag,value} were updated. Since S_GET_VALUE() can't be used to
retrieve what used to be dot_value, introduce a new helper to fetch both
frag and offset.