libdwarf: Add some constants from DWARF 5

This is not exhaustive - DWARF 5 has some new enumeration types not
implemented here - but I think I caught all the ones that are extended
in DWARF 5, plus the new compilation unit type (DW_UT_*), needed when
parsing .debug_info headers.

These were useful when extending libdwarf/ctfconvert/readelf to handle
DWARF generated by gcc 12, which is version 5 by default.

Reviewed by: emaste
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38273