Updated usdt_sample for ubuntu 21.10 (and python3)

Updated usdt_sample for ubuntu 21.10 (and python3)

* Updated readme for Ubuntu 21.10 and python3.
* Readability improvements.
* Using atomic_increment (thus requires bcc 0.21+).
* Using/tested probes created using systemtap (dtrace), next to the
probes created with macros in the folly header.
* Also tested while building usdt_sample with clang.
* Some (modern) cmake changes.
* Added script to execute build and sample python scripts.