First attempt at some performance tracking tools. Includes a script vg_perf

First attempt at some performance tracking tools.  Includes a script vg_perf
(use "make perf" to run) that executes test programs and times their
slowdowns under various tools.  It works a lot like the vg_regtest script.
It's a bit rough around the edges -- eg. you can't currently directly
compare two different versions of Valgrind, which would be useful -- but it
is a good start.

There are currently two test programs in perf/.  More will be added as time
goes on.  This stuff will be built on so that performance changes can be
tracked over time.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@5323