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

Closes #650

It’s not clear if these values are used or needed by the simulator. The doc only seems to mention arm values for the actual device:
https://developer.apple.com/library/content/documentation/General/Reference/InfoPlistKeyReference/Articles/iPhoneOSKeys.html#//apple_ref/doc/uid/TP40009252-SW3

Pass the tests only if we find an expected positive output (rather than checking for negative output to fail the tests).

Documentation generation appears to be failing on master (https://travis-ci.org/rust-lang/libc/jobs/183483333):

```
Collecting ghp-import
  Downloading ghp-import-0.4.1.tar.gz
Collecting travis
  Could not find a version that satisfies the requirement travis (from versions: )
No matching distribution found for travis
```

Basically `--user` doesn't appear to take an option, so `pip install ghp_import --user $USER` makes pip think the user is a package that needs to be installed (in this case `travis`). As there is no `travis` package, it dies.

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.

Also cache on appveyor to speed up builds.

* Override the compiler via CC; ./configure can't seem to detect it.
* Unset CROSS_COMPILE when running make. Per the comment, we otherwise
  end up invoking commands like i686-ar.

Specifically:

* Update the base docker images to ubuntu 16.10
* Update musl to version 1.1.15

This is necessary since the versions we were previously using do not
define CLONE_NEWCGROUP, so adding the symbol to the libc crate causes
test failures.

and fix the statvfs struct for this target

also "backport" two glibc fixes:

- ipc_perm.mode is c_uint. [commit][1] [discussion][2]
- bump SIGSTKSZ to 16384. [commit][3] [discussion][4]

so libc-test passes.

[1]: https://sourceware.org/git/?p=glibc.git;a=commit;h=5c40c3bab2
[2]: https://www.sourceware.org/ml/libc-alpha/2014-10/msg00446.html
[3]: https://sourceware.org/git/?p=glibc.git;a=commit;h=b763f6ae859ecea70a5dacb8ad45c71d5f667e2e
[4]: https://sourceware.org/ml/libc-alpha/2015-04/msg00033.html

closes #231

- 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

Use the standard mips compilers from Ubuntu now