- add #[no_mangle] to the panic_fmt lang item to adjust to changes in
the visibility algorithm
- adjust to changes in the layout of Cargo's target directory
- use a newer Xargo to reduce the build time of the sysroot (only core
is compiled as part of the sysroot now)
due to how Cargo works, the rustc-cfg in build.rs was picking our target
specifications rather than the one the user meant to use. Placing our
target files in any place other than the root of the Cargo project
avoids the issue.
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
