Add support for CloudABI.

CloudABI is a sandboxed UNIX-like runtime environment, based on the
principle of capability-based security. As CloudABI is intended to be
cross-platform, the system call layer is specified here:

    https://github.com/NuxiNL/cloudabi/blob/master/cloudabi.txt

From these definitions, we automatically generate C and Rust bindings.
The latter is published on crates.io:

    https://crates.io/crates/cloudabi

My goal is to implement libstd for CloudABI in such a way that it uses
the C library as little as possible; only in places where it would ease
interfacing with C code (e.g., thread creation). In places where
constants in the C library are directly based on the CloudABI
specification (e.g., errnos), use the constants provided by the cloudabi
crate.