valgrind: Disable on Risc-V

There are no defined instruction sequences in include/valgrind.h for
Risc-V, so CONFIG_VALGRIND will do nothing on this arch (and possibly won't
compile?). Update Kconfig accordingly.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>