It wasn't expecting a `*.d` file to exist, now it does.

This works by specifying a "runner" for actually executing the binary.
This doesn't apply to the Android or NetBSD runs because there
isn't a simple binary that just runs the executable.

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.

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

Closes #650

Also cache on appveyor to speed up builds.

- allow QEMU name to have subdirectory inside (replace `/` by `__` in
  the filename)
- add a new code path for plain qcow2 image (for efficient bandwidth
  usage, the qcow2 is expected to use compressed qcow2 format)
- move freebsd image to explicitly download the gzipped QEMU

Download a trunk OpenWRT toolchain and verify bindings against that. Also fixup
a few stray errors today

Previously we were somewhat half-Docker, half-Travis, half apt-get, etc. This
commit alters the CI infrastructure to use Docker images for each target. This
should make it much easier to update the images and tweak various bits and
pieces of installed software. Additionally, it's also much clearer now what's
needed for each suite of tests!

Some images were updated a bit (e.g. musl is now 1.1.14), but other images
encountered failures when updating so they're not getting updated just yet.

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.