[RFA] Add dwarf4 values to include/dwarf2.h

[Cary Coutant]

>> This patch adds various dwarf4 elements, plus DW_AT_GNU_odr_signature,
>> to include/dwarf2.h.
>
> Should this file even be called dwarf2.h any more?

DWARF-2 was so different from DWARF-1, it was essentially a new
format, so it made sense at the time. DWARF-3 and DWARF-4 are just
mostly-compatible extensions of DWARF-2, and updating the name for
each new revision would only be counterproductive. Just think of it as
"dwarf2+.h".

-cary