scripts/sortextable: support objects with more than 64K sections.

Building with a large config and -ffunction-sections results in a large
number of sections and sortextable needs to be able to handle that.
Implement support for > 64K sections as modpost does.

Signed-off-by: Jamie Iles <jamie.iles@oracle.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>