elflint: Add _edata and _end to the list of possibly dangling gnuld symbols.

gnuld might keep dangling/unused symbols around (see sourceware PR13621).
Add _edata (end of initialized data address) and _end (end of uninitialized
data address) to that list. They might have been added by gnuld even if
there is no such [un]initialized data. Also add the double underscore
variants which are used on some architectures.

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