Closes #736

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!

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

https://github.com/kripken/emscripten/issues/5288 is fixed now.

A few changes have necessitated some updates on our end

This helps caching between runs and can help speed up turnaround time for
various operations. The old android container didn't work out for some reason
due to permissions so the definition has now been vendored locally to just
rebuild it each time.

Should help us keep the size explosion under control.

and fix the statvfs struct for this target

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.

I only added "*at" syscall relative to Posix2008 specification.

At the moment OSX and Android are partially supporting this API.

This include:
* `openat`
* `faccessat`
* `fchmodat`
* `fchownat`
* `fstatat`
* `linkat`
* `mkdirat`
* `mknodat` [0]
* `readlinkat`
* `renameat`
* `symlinkat`
* `unlinkat`
* `mkfifoat` [0][1]

[1] Not available on Android at the moment:
* `mkfifo`

[1] Not available on OSX at the moment:
* `mkfifoat`
* `mknodat`

And exclude operating system specific API:
* `renameat2`: Linux specific
* `execveat`: Linux specific since 3.19
* `futimesat`: GNU extension
* `name_to_handle_at`: GNU extension
* `scandirat`: GNU extension
* `fanotify_mark`: belong to fanotify API.

Only running them against auto-libc unfortunately means that PRs don't run CI.

iOS was fixed awhile back and we should have rumprun nightlies now, so let's use
them!

It's tough to have PRs bounce or to have a back and forth with contributors
about minor style quibbles. Sometimes it ends up just being easier to fix style
after the fact, but let's add some automation to help this!

This commit adds a script to run on CI and locally to verify the style of this
repository. There's a few stylistic guidelines to ensure that definitions are
understandable across the jungle of modules. This consistency should help assist
readability for any future readers!