dwarflint: check_linkage_external_die keep symbol table and double check.

dwarflint: check_linkage_external_die keep symbol table and double check.

We pull the symbol table from the dwarf file and check when a linkage_name
is set that the symbol table contains that name and is marked not marked
local, with the following exceptions:

- No symbol in table, OK, if not a defining or const object.
  Or GNU extension, anonymous structs can have a linkage_name.
- Symbol in table marked local, OK if not a defining object
  and marked external.  Which means it comes from some external
  symbol table.