Using __libdw_formptr from C++

[Petr Machata <pmachata at redhat dot com>]

Hi there,

this is about https://bugzilla.redhat.com/show_bug.cgi?id=679815 that
Mark filed this week.  I fixed on dwarf branch a couple bugs that
dwarflint was hitting, but there's one outstanding issue that I don't
want to commit to dwarf.

The thing is that in range_list::const_iterator we now use
dwarf_formudata.  dwarf_formudata doesn't handle DW_FORM_sec_offset, and
I'm not sure that it should.  The function that does handle them is
__libdw_formptr and indeed where the iterators use dwarf_formudata, one
finds a comment in the spirit of "xxx __libdw_formptr".  So I rewrote
the code to use that instead, just to find at link time that the
function obviously is not public, and since we compile the C++ bits
separately through an .a archive, this bites us.

Well the logic is written now, and seems to work, so I've committed it,
just separately to pmachata/range_list_iterator_formptr.  The hope is
that it will be easy enough to adjust the patch to whatever the final
solution might be.  I don't suppose that bundling C++ bits with C bits
in the single DSO ever was in plan, so some sort of publishing useful,
not-quite-ABI bits from libdw.so will be necessary.

Thanks,
PM