Genericize addrmap handling in the DWARF index writer

This updates the DWARF index writing code to make the addrmap-writing
a bit more generic.  Now, it can handle multiple maps, and it can work
using the maps generated by the new indexer.

Note that the new addrmap_index_data::using_index field will be
deleted in a future patch, when the rest of the DWARF psymtab code is
removed.