Add ubpf JLL, use in tests

[jpsamaroo]

ubpf is a great Apache-licensed eBPF interpreter, and is easy to use for testing BPF program output. We should pack it into a JLL, export some functions for using the VM, and use it for testing codegen. It doesn't have many of the features in the Linux kernel's BPF interpreter (yet), but it also doesn't require us to have root permission to test with, and doesn't have inconsistent feature support across kernel versions.