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 <john.g.garry@oracle.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jing Zhang <renyu.zj@linux.alibaba.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Oliver Sang <oliver.sang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Philip Li <philip.li@intel.com>
Cc: Sandipan Das <sandipan.das@amd.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: Xu Yang <xu.yang_2@nxp.com>
Link: https://lore.kernel.org/r/20240730191744.3097329-3-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>