classes/meson: Add optional rust definitions

Adds the rust tools to the cross and native files if present so that
projects that use both rust and meson can build

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 0ec40fa3)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>