valgrind: allow selecting which tools to install

The full valgrind installation takes more than 20 MB, while one typically does
not use all of its tools. This patch adds extra config options to select which
tools to install.

[Peter: Don't default experimental tools to 'y' as suggested by Arnout]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>