unstrip: Also check sh_size in compare_unalloc_sections.

unstrip: Also check sh_size in compare_unalloc_sections.

compare_unalloc_sections only checked sh_flags and the section names.
This would cause stripped/debug section mismatches when there were
multiple sections with the same name and flags. Fix this by also checking
the size of the section matches.

Add a testcase that has two ".group" sections created on i386 with the
gcc annobin plugin.

Signed-off-by: Mark Wielaard <mark@klomp.org>