Rust helloworld example application

[huyhoang8398]

add Rust helloworld programs

[razvand]

LGTM. Thanks, @huyhoang8398

[mkroening]

This App does not work on App ELF loader yet.

Apart from the Makefile not working, the resulting binary fails with

fatal runtime error: assertion failed: signal(libc::SIGPIPE, libc::SIG_IGN) != libc::SIG_ERR

It works on Linux, though.

The failing assert is here: std/src/sys/unix/mod.rs#L155

[mkroening]

Without removing the trailing colon I get

$ make
Makefile:3: *** target pattern contains no '%'.  Stop.

Suggested change

	helloworld: helloworld.rs:
	helloworld: helloworld.rs

[razvand]

@huyhoang8398 , @mkroening , I'm not able to get it to build. I get the error:

error[E0463]: can't find crate for `std`
  |
  = note: the `x86_64-unknown-linux-musl` target may not be installed
  = help: consider downloading the target with `rustup target add x86_64-unknown-linux-musl`

I've little experience with Rust and I aimed to solve this by using rustup but I wasn't able to.

Could you please add a README.md file describing what needs to be installed / set up for this to work? Also, please add the helloworld executable file in the commit, a clean rule in the Makefile and, if required, a .gitignore file. See the support for other helloworld programs (such as C).

[razvand]

This is superseded by #8 from @mkroening