notable changes in the docker-based testing infrastructure
- the docker containers can now modify $CARGO_HOME, to re-use the outer
Cargo registry, and the target directory to re-use build artifacts.
- the docker containers are removed when their execution finishes
because it's no longer necessary to re-start them to inspect them
because all the interesting output is in the outer target directory
This commit moves over most of the testing infrastructure to in-tree docker
images that are all dispatched to from Travis (no other test configuration).
This allows versioning modifications to the test infrastructure as well as the
code itself. Additionally separate docker images allows for easy modification of
one without worrying about tampering of others as well as easy addition of new
targets by simply adding a new `Dockerfile`.
Additionally this commit bundles the master version of the `compiler-rt` source
repository from `llvm-mirror/compiler-rt` to test against. The compiler-rt
library itself is compiled as a `cdylib` which is then dynamically located at
runtime and we look for symbols in. There's a few hoops here, but they currently
get the job done.
All tests now execute against both gcc_s and compiler-rt, and this
testing strategy is now all hidden behind a macro as well (refactoring
all existing tests along the way).
