scripts: add stackdelta script

This adds a simple perl script for reading two files as produced by
the stackusage script and computing the changes in stack usage. For
example:

$ scripts/stackusage -o /tmp/old.su CC=gcc-4.7 -j8 fs/ext4/
$ scripts/stackusage -o /tmp/new.su CC=gcc-5.0 -j8 fs/ext4/
$ scripts/stackdelta /tmp/{old,new}.su | sort -k5,5g

shows that gcc 5.0 generally produces less stack-hungry code than gcc
4.7. Obviously, the script can also be used for measuring the effect
of commits, .config tweaks or whatnot.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Michal Marek <mmarek@suse.com>