Add Redox to cross compile CI

[jackpot51]

This will require rust-lang/libc#2728 before it is functional

[jackpot51]

Here is the error:

error[E0554]: `#![feature]` may not be used on the stable release channel
  --> /home/vsts/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/libc-0.2.120/src/lib.rs:29:5
   |
29 |     feature(static_nobundle, native_link_modifiers, native_link_modifiers_bundle)
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0554`.
error: could not compile `libc` due to previous error
warning: build failed, waiting for other jobs to finish...
error: build failed
##[error]Bash exited with code '101'.

I will mark this PR as ready for review when I can update libc to fix this error

[jackpot51]

Now that libc 0.2.121 is released, this should probably work, I have marked it as ready for review.

[Thomasdezeeuw]

Thanks @jackpot51