Test ring on -*sel4* targets

[nspin]

The purpose of this PR is to get the ring crate compiling for *-sel4* targets and to run its unit tests on those targets.

First off, configuring upstream ring for certain baremetal targets is currently broken. For now, we will use coliasgroup/ring@10a2b3c, but I will propose a more suitable solution upstream.

Second, the test crate (libtest) that ships with the Rust toolchain requires std. I've discovered that you can run unit tests written for libtest by substituting an API-compatible crate with something like :

[dev-dependencies]
test = { package = "my-fake-test-harness", path = "../my-fake-test-harness" }

I also had to patch ring's tests to use no_std.

This PR uses this ring tree: https://github.com/coliasgroup/ring/tree/12eafcfa062e6713b6ee2123d90ca2a48bece768

Only the first patch mentioned will be necessary for using ring in this project, the others are just for running its unit on *-sel4* targets.