dwarf.h: Add DWARF v6 langauge attributes and DW_LNAME constants

dwarf.h: Add DWARF v6 langauge attributes and DW_LNAME constants

https://dwarfstd.org/languages-v6.html defines the DWARF v6 language
attributes, DW_AT_language_name, DW_AT_language_version and DW_LNAME
constants as usable by pre-DWARF v6 producers and consumers.

Also add new DW_LANG_V and DW_LANG_Algol68 (DWARFv5) language constants.

    * config/known-dwarf.awk: Handle DW_LNAME.
    * libdw/dwarf.h: Add DW_AT_language_name and DW_AT_language_version
    plus all currently defined DW_LNAME constants. Add DW_LANG_V and
    DW_LANG_Algol68.
    * libdw/dwarf_default_lower_bound.c: Add DW_LANG_V and
    DW_LANG_Algol68.

Signed-off-by: Mark Wielaard <mark@klomp.org>