scripts: add stackusage script

The current checkstack.pl script has a few problems, stemming from the
overly simplistic attempt at parsing objdump output with regular
expressions: For example, on x86_64 it doesn't take the push
instruction into account, making it consistently underestimate the
real stack use, and it also doesn't capture stack pointer adjustments
of exactly 128 bytes [1].

Since newer gcc (>= 4.6) knows about -fstack-usage, we might as well
take the information straight from the horse's mouth. This patch
introduces scripts/stackusage, which is a simple wrapper for running
make with KCFLAGS set to -fstack-usage. Example use is

scripts/stackusage -o out.su -j8 lib/

The script understands "-o foo" for writing to 'foo' and -h for a
trivial help text; anything else is passed to make.

Afterwards, we find all newly created .su files, massage them a
little, sort by stack use and write the result to a single output
file.

Note that the function names printed by (at least) gcc 4.7 are
sometimes u...