Since these are defined in C as macros, they must be reimplemented in
libc as Rust functions.  They're hard to get exactly right, and they
vary from platform to platform.  The test builds custom C code that uses
the real macros, and compares its output to the Rust versions' output
for various inputs.

Skip the CMSG_NXTHDR test on sparc64 linux because it hits a Bus Error.

Issue #1239

Skip the entire cmsg test program on s390x because it dumps core
seemingly before the kernel finishes booting.

Issue #1240

We now create an additional binary `linux_fcntl` for testing this
since there are header conflicts when including all necessary headers.
This binary is run on all platforms even though it's empty on all non-
Android/non-Linux platforms.

Testing has been switched from a custom binary to using a runner-less
test (or pair of tests). This means that for local development a simple
`cd libc-test && cargo test` will run all the tests. CI has also been
updated here to reflect that.

It's now broken due to changes in the `gcc` crate and having a too-old compiler,
and in general it's unfortunately architecturally so different from the other
test frameworks that it's difficult to maintain over time.

This commit adds support to test all libc definitions on both OpenBSD and
FreeBSD via QEMU userspace emulation. Specially prepared images for each OS are
used which are essentially intended to run a script on startup and then exit.
Documentation has been added to the `ci/README.md` file describing this new
system.

Extracted tests!