perf-urgent-2020-04-05

 core:

   - Support for cgroup tracking in samples to allow cgroup based
     analysis

 tools:

   - Support for cgroup analysis

   - Commandline option and hotkey for perf top to change the sort order

   - A set of fixes all over the place

   - Various build system related improvements

   - Updates of the X86 pmu event JSON data

   - Documentation updates