perf jevents: Autogenerate empty-pmu-events.c

empty-pmu-events.c exists so that builds may occur without python
being installed on a system. Manually updating empty-pmu-events.c to
be in sync with jevents.py is a pain, let's use jevents.py to generate
empty-pmu-events.c.

1) change jevents.py so that an arch and model of none cause
   generation of a pmu-events.c without any json. Add a SPDX and
   autogenerated warning to the start of the file.

2) change Build so that if a generated pmu-events.c for arch none and
   model none doesn't match empty-pmu-events.c the build fails with a
   cat of the differences. Update Makefile.perf to clean up the files
   used for this.

3) update empty-pmu-events.c to match the output of jevents.py with
   arch and mode of none.

Committer notes:

The firtst paragraph is confusing, so I asked and Ian further clarified:

 ---
The requirement for python hasn't changed.

Case 1: no python or NO_JEVENTS=1
Build happens using empty-pmu-events.c that is checked in, no python
is required.

Case 2: python
pmu-events.c is created by jevents.py (requiring python) and then built.
This change adds a step where the empty-pmu-events.c is created using
jevents.py and that file is diffed against the checked in version.

This stops the checked in empty-pmu-events.c diverging if changes are
made to jevents.py. If the diff causes the build to fail then you just
copy the diff empty-pmu-events.c over the checked in one.
 ---

Reviewed-by: John Garry <[email protected]>
Signed-off-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jing Zhang <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Oliver Sang <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Philip Li <[email protected]>
Cc: Sandipan Das <[email protected]>
Cc: Weilin Wang <[email protected]>
Cc: Xu Yang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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