perf pmu-events: Parallelize JSON and metric pre-computation in jevents.py

Currently, jevents.py parses hundreds of JSON event and metric files
sequentially across all CPU architectures during Kbuild startup,
taking ~3.5 seconds of single-core execution time.

Refactor jevents.py to pre-populate its internal JSON AST cache in
parallel across all available CPU cores using
ProcessPoolExecutor. First gather all the paths with ftw and
collect_json, then spawn _parallel_read_json_events that starts
workers to just read the json events. Define the worker process
initializer _init_worker so that _arch_std_events is available under
spawn multiprocessing semantics.

This accelerates the JSON parsing phase by over 10x (from ~3.0s down
to ~290ms), reducing overall jevents.py execution time by 3.5x (from
~3.56s down to ~1.03s).

Tested-by: James Clark <james.clark@linaro.org>
Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

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