Tamir Duberstein 8c61fc9ea6
integration-test: compile C probes using build.rs
- Add libbpf as a submodule. This prevents having to plumb its location
  around (which can't be passed to Cargo build scripts) and also
  controls the version against which codegen has run.
- Move bpf written in C to the integration-test crate and define
  constants for each probe.
- Remove magic; each C source file must be directly enumerated in the
  build script and in lib.rs.
..
ext.bpf.c integration-test: compile C probes using build.rs
main.bpf.c integration-test: compile C probes using build.rs
multimap-btf.bpf.c integration-test: compile C probes using build.rs
text_64_64_reloc.c integration-test: compile C probes using build.rs