This commit adds CI for a few more targets:
* i686-unknown-linux-gnu
* arm-unknown-linux-gnueabihf
* armv7-unknown-linux-gnueabihf
* aarch64-unknown-linux-gnu
The CI here is structured around using a Docker container to set up a test
environment and then QEMU is used to actually execute code from these platforms.
QEMU's emulation actually makes it so we can continue to just use `cargo test`,
as processes can be spawned from QEMU like `objdump` and files can be read (for
libbacktrace). Ends up being a relatively seamless experience!
Note that a number of intrinsics were disabled on i686 because they were failing
tests, and otherwise a few ARM touch-ups were made to get tests passing.
