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.

* Add syscall tables to most remaining arches in `src/unix/notbsd`
* Add aarch64/arm musl to CI
* Update dependencies

- add DCCP constant definitions
- add clock_gettime related functions to macOS
- add fstatat64 on linux/android
- add preadv64/pwritev64 on linux/android
- add utimensat on solaris, netbsd and openbsd
- add IP_BINDANY on freebsd

Rebase of #610 and also move emscripten up much higher in the hierarchy to
ensure that it doesn't have too much of a ripple effect on other platforms.

This involved moving down a good number of definitions, but hopefully was done
with care to not break anything!

We can't test s390x because qemu segfaults but we can at least verify that it
compiles.

Closes #650

Forgot to update cargo.lock before prior commit.

closes https://github.com/rust-lang/libc/issues/539

Also cache on appveyor to speed up builds.