delay element reindexing and symbol reordering

Right now, reindexing of the included sections and symbols is done
when they migrate to the output kpatch_elf structure.  However, due
to recently added features, the section and symbol list is not
final at this point, leading to constant tracking of the indexes for
addition sections and symbols added after this point.  Additionally,
symbols have to be in a particular order, adding to the complexity.

This commit delays the reindexing and symbol reordering until the
section and symbol lists are finalized, removing the need to
track indexes and placeholders in the symbol list.

Signed-off-by: Seth Jennings <sjenning@redhat.com>