unstrip: Handle debuginfo files with missing SHF_INFO_LINK section flags.

unstrip: Handle debuginfo files with missing SHF_INFO_LINK section flags.

With GCC 5 there might be a .rela.plt section with SHF_INFO_LINK set.
Buggy binutils objdump might strip it from the section in the debug file.
Ignore such differences for relocation sections and put the flag back
if necessary.

Also improve the error message a little by only discarding the already
matched sections if there is an prelink undo section. Otherwise we will
report all sections as not matching if the file wasn't prelinked instead
of just the non-matching sections.

New testfiles generated by gcc5 and binutils objdump added.

Signed-off-by: Mark Wielaard <mjw@redhat.com>