perf jevents: Support copying the source json files to OUTPUT

perf jevents: Support copying the source json files to OUTPUT

The jevents command expects all json files to be organized under a
single directory. When generating json files from scripts (to reduce
laborious copy and paste in the json) we don't want to generate the
json into the source directory if there is an OUTPUT directory
specified. This change adds a GEN_JSON for this case where the
GEN_JSON copies the JSON files to OUTPUT, only when OUTPUT is
specified. The Makefile.perf clean code is updated to clean up this
directory when present.

This patch is part of:
https://lore.kernel.org/lkml/20240926173554.404411-12-irogers@google.com/
which was similarly adding support for generating json in scripts for
the consumption of jevents.py.

Tested-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>