This commit adds a `ci/build.sh` script that checks that libc builds correctly
for some common configurations (`--no-default-features`, `default`,
`extra_traits`) on most targets supported by Rust since Rust 1.13.0 (the oldest
Rust version that libc supports).

The build matrix is refactored into two stages.

The first stage is called `tools-and-build-and-tier1` and it aims to discover
issues quickly by running the documentation and linter builds, as well as
checking that the library builds correctly on all targets in all supported
channels and "problematic" Rust versions; Rust versions adding major new
features like `repr(align)`, `union`, etc. This first stage also runs
libc-test for the tier-1 targets on linux and osx. These builds finish
quickly because no emulation is necessary.

The second stage is called `tier2` and it runs libc-test for all other targets
for which we are currently able to do so.

Closes #1229 .

Closes #1170 .

That service no longer exists.

Fixes #1163

This bug was never diagnosed.  Perhaps it's been resolved by a newer
compiler.
Fixes #856

This patch removes the x86_64-rumprun-netbsd triple from Travis CI.

Closes #1064 .

Looks like Travis has moved on from our old images, so we're forced to
update.

Apparently it fails on 1.23.0 for mysterious reasons

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

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!