Add a check that some crates build with a stable toolchain.
Add a check that some crates build with a stable toolchain.

We want `libtock-rs` to build on a stable toolchain in the future, once `asm!` is stable. Some of `libtock-rs`' crates (particularly `libtock_platform` and `libtock_unittest`) already build with a stable toolchain. This adds a check to `make test` that verifies those crates compile with a stable toolchain.

tock/libtock-rs