scripts: fix faddr2line to work on last symbol

If faddr2line is given a function name which is the last one listed by
"nm -n", it will fail because it never finds the next symbol.

So teach the awk script to catch that possibility, and use 'size' to
provide the end point of the last function.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>