rust-bin: new package

This package provides a pre-built version of rustc, the compiler for the
Rust programming language, fetched from the upstream project.

A pre-built version of the standard library for the host as well as one
for the chosen target are also fetched and installed.

Only the host variant is provided to allow the user to cross-compile
Rust programs and run them on the target.

This package could also be used to provide a bootstrap compiler when building
Rust from source. So, in order to add it as a build dependency, the compiler and
standard libraries are only installed in $(HOST_DIR) if the package is
explicitly selected.

The menuconfig entry for rustc is also updated to expose this provider.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>