Pass configuration flags to `cargo`

As of #2822 "Start reusing rustc's format_args parser", we invoke cargo to produce object files to be linked into GCC. We currently don't specify any compilation flags and linker to use, so it's easy to run into mismatches between what GCC is configured for (for example, specific linker), and what cargo uses by default (for example, system cc for linking).

For example, the linker may be specified: https://doc.rust-lang.org/cargo/reference/config.html#targettriplelinker -- but do we need the Rust target triple for that (see #2898)? Trying to use a cfg() expressions à la --config "target.'cfg(all())'.linker=[...]" only produces:

warning: unused key `linker` in [target] config table `cfg(all())`

(Maybe I've misunderstood how that is to be used.)

Start by inspecting the cargo invocation introduced by #2822 and compare it with the Rust target-triple questions in #2898. Read Cargo's target linker configuration documentation linked in the issue. Done means cargo receives the relevant compilation flags and linker settings used by GCC's configuration, avoiding mismatches.