Support DWARF4 linkage names

Support DWARF4 linkage names

With -gdwarf-4, gcc outputs DW_AT_linkage_name instead of
DW_AT_MIPS_linkage_name.  Semantics are unchanged.

* dwarf_wrappers.h (dwarf_linkage_name): New, retrieves the linkage_name
  by either the old or new attribute value.
* dwflpp.cxx (dwflpp::iterate_over_functions): Use dwarf_linkage_name.
  (dwflpp::vardie_from_symtable): Ditto.
* tapsets.cxx (dwarf_query::add_probe_point): Ditto.