Similar to 3b783652cc, this makes sure
rust-native does not depend on target variables. This allow one
rust-native to be shared between machines of different architectures
again.
For the record, the following guide was used to find out why
do_rust_gen_targets differed between different machines.
https://wiki.yoctoproject.org/wiki/TipsAndTricks/Understanding_what_changed_(diffsigs_etc)