Add Rust hello-no-std example

[mkroening]

Closes https://github.com/unikraft/static-pie-apps/pull/3.

This example builds a freestanding Rust application which manually performs Linux syscalls.

This requires rust-std (only core though) for x86_64-unknown-none. Those binaries are not yet available on stable.

You can default to nightly and install the binaries like this:

$ rustup install nightly
$ rustup default nightly
$ rustup target install x86_64-unknown-none

[mkroening]

Closing in favor of https://github.com/unikraft/static-pie-apps/pull/3, since we want a normal Rust application to run. Was fun to do everything manually, though. :D

CC: @razvand