rust: fix issue building cross-canadian tools for aarch64 on x86_64

rust: fix issue building cross-canadian tools for aarch64 on x86_64

Commit bd36593ba3db758b3eacc974e48468a665967961 did introduce a
regression when building package rust-cross-canadian-aarch64
on a x86_64 host. This commit will fix that configuration.

Suggested-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Peter Bergin <peter@berginkonsult.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>