Rollup merge of #47427 - EdSchouten:cloudabi-ci, r=alexcrichton

Rollup merge of #47427 - EdSchouten:cloudabi-ci, r=alexcrichton

Add a Docker container for doing automated builds for CloudABI.

Setting up a cross compilation toolchain for CloudABI is relatively
easy. It's just a matter of installing a somewhat recent version of
Clang (5.0 preferred) and installing the corresponding
`${target}-cxx-runtime` package, containing a set of core C/C++ libraries
(libc, libc++, libunwind, etc).

Eventually it would be nice if we could also run `x.py test`. That,
however still requires some more work. Both libtest and compiletest
would need to be adjusted to deal with CloudABI's requirement of having
all of an application's dependencies injected. Let's settle for just
doing `x.py dist` for now.