As discussed in rust-lang#2447, rustup versions are getting closer to rustc
versions (because the rustup maintainers are working too much I'm sure).
This leads to confusion when running `rustup --version` while intending
to run `rustc --version`.

The change introduced here will print two additional (stderr) lines
after rustup's default `--version` output. The first mentions this
issue, and the second will print the output of `rustc --version` as a
hint (by trivially calling out to `rustc`, whatever binary that may be).