bcc release v0.19.0

  * Support for kernel up to 5.11
  * allow BCC as a cmake subproject
  * add LPORT support in tcpconnlat and tcpconnect
  * added bpf_map_lookup_and_delete_batch support
  * new tools: virtiostat
  * new libbpf-tools: cpufreq, funclatency, cachestat
  * add install target to libbpf-tools
  * a few lua fixes
  * doc update and bug fixes