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!

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