[3/4] RFC: add DWARF index support

[Tom Tromey]

Tom> Another idea is to make the user use objcopy to put the new data into
Tom> the objfile.  I am not very fond of this, though.  In my experiments I
Tom> think objcopy had bugs dealing with separate debug files.

For the record, we are addressing this by adding the index to the
objfile before stripping debuginfo.  There's a patch in the elfutils
strip now that recognizes ".gdb_index" as a debug section.

Tom