dbghelp: implement DW_TAG_unspecified_type

based on dwarf2_parse_base_type

Signed-off-by: David Heidelberger <david.heidelberger@ixit.cz>
winedbg: list Dwarf2 GNU extensions as TODO to workaround spam in debug

Signed-off-by: David Heidelberger <david.heidelberger@ixit.cz>