mirrors.bbclass: use shallow tarball for binutils-native

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

The original BB_GIT_SHALLOW lines were added because of https://www.mail-archive.com/yocto@lists.yoctoproject.org/msg08752.html



Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>