Summary: Add a submodule for bpftrace, and modify the build system to build it.

Test Plan: Count on phabricator to catch any build issues!

Reviewers: #engineering, zasgar

Reviewed By: #engineering, zasgar

Differential Revision: https://phab.corp.pixielabs.ai/D292

GitOrigin-RevId: 6fce53a1c509dd9511f03de60b727b94d59a5440