`rustc.config` is called `rust.rustcTarget` now, and
`{rustc -> rust}.platform`.
This is the new way (tm), and is preferred since
https://github.com/NixOS/nixpkgs/pull/271707 -
though the documentation still is outdated, and some expressions in
nixpkgs were using the old interface.
This updates both.
stdenv.targetPlatform really shouldn't be used by software that
doesn't generate or manipulate binaries. I reviewed all uses of
targetPlatform outside of pkgs/development/compilers and pkgs/stdenv
and replaced those which weren't involved in something which fits
these criteria.
