mirrors.bbclass: use shallow tarball for nativesdk-binutils

This is useful e.g. when using meta-clang, which introduces a dependency to nativesdk-binutils when
populating the SDK, and then a full tarball of binutils is fetched additionally to a shallow tarball.

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>