To cross-compile from x86_64 to armv7hf, use:
$ sudo apt install gcc-7-multilib-arm-linux-gnueabihf
and then add in ~/.cargo/config:
[target.armv7-unknown-linux-gnueabihf] linker = "arm-linux-gnueabihf-gcc-7"
to install and able the armv7 compiler
Finally add the linker for armv7 architecture with:
$ rustup target add armv7-unknown-linux-gnueabihf
To build the project for the armv7 target use:
$ cargo build --target=armv7-unknown-linux-gnueabihf