Modelled after the cmsg tests, this wraps the C macro into a function, and then
compares the results to the Rust implementation in libc.

Related to #657 .

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

Bumps [ctest](https://github.com/alexcrichton/ctest) from 0.2.7 to 0.2.8.
- [Release notes](https://github.com/alexcrichton/ctest/releases)
- [Commits](https://github.com/alexcrichton/ctest/commits

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [ctest](https://github.com/alexcrichton/ctest) from 0.2.6 to 0.2.7.
- [Release notes](https://github.com/alexcrichton/ctest/releases)
- [Commits](https://github.com/alexcrichton/ctest/commits/0.2.7

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [ctest](https://github.com/alexcrichton/ctest) from 0.2.4 to 0.2.6.
- [Release notes](https://github.com/alexcrichton/ctest/releases)
- [Commits](https://github.com/alexcrichton/ctest/commits

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

There are none of them in `libc` except for `__progname` on Android, but
that one cannot be tested because it's not present in any header files.

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

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 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.

Extracted tests!

Running a test per thread really doesn't play well with QEMU emulation, so just
make it easy on ourselves and don't run threads.