This will let us add big endian testing to CI for our C code. (We were
already doing it for our Rust code.)
This is adapted from test_vectors/cross_test.sh. It works around the
limitation that the `cross` tool can't reach parent directories. It's an
unfortunate hack, but at least it's only for testing. It might've been
less hacky to use symlinks for this somehow, but I worry that would
break things on Windows, and I don't want to have to add workarounds for
my workarounds.
Jack O'Connor