dwarflint: linkage_name attributes checking.

dwarflint: linkage_name attributes checking.

Dwarf4 introduces DW_AT_linkage_name, which is like DW_AT_MIPS_linkage_name
already used by gcc for earlier versions. Mark DW_TAG_common_block,
DW_TAG_constant, DW_TAG_entry_point, DW_TAG_subprogram and DW_TAG_variable
as optionally having at_linkage_name (either the old or new variant) in
expected-at.cc. Add new check_linkage_external_die.cc to find DIEs with
linkage_name set, but not marked DW_AT_external.