`cargo` should build for the host system

[tschwinge]

As of #2822 "Start reusing rustc's format_args parser", we invoke cargo to produce object files to be linked into GCC. I've not yet run into the following -- but I suspect that cargo by default builds for the system it's currently running on: the GCC build system. However, these object files are to be linked into the compiler we're building, the GCC host system.

Therefore, I suspect, we should pass:

      --target <TRIPLE>         Build for the target triple

In the Makefile, we do have the GCC host system available in $(host) (or is $(host_noncanonical) applicable here?) -- but how to programmatically translate that into the corresponding Rust target triple, https://doc.rust-lang.org/cargo/appendix/glossary.html#target, https://doc.rust-lang.org/nightly/rustc/platform-support.html?